std: Add a new `env` module

This is an implementation of [RFC 578][rfc] which adds a new `std::env` module
to replace most of the functionality in the current `std::os` module. More
details can be found in the RFC itself, but as a summary the following methods
have all been deprecated:

[rfc]: https://github.com/rust-lang/rfcs/pull/578

* `os::args_as_bytes`   => `env::args`
* `os::args`            => `env::args`
* `os::consts`          => `env::consts`
* `os::dll_filename`    => no replacement, use `env::consts` directly
* `os::page_size`       => `env::page_size`
* `os::make_absolute`   => use `env::current_dir` + `join` instead
* `os::getcwd`          => `env::current_dir`
* `os::change_dir`      => `env::set_current_dir`
* `os::homedir`         => `env::home_dir`
* `os::tmpdir`          => `env::temp_dir`
* `os::join_paths`      => `env::join_paths`
* `os::split_paths`     => `env::split_paths`
* `os::self_exe_name`   => `env::current_exe`
* `os::self_exe_path`   => use `env::current_exe` + `pop`
* `os::set_exit_status` => `env::set_exit_status`
* `os::get_exit_status` => `env::get_exit_status`
* `os::env`             => `env::vars`
* `os::env_as_bytes`    => `env::vars`
* `os::getenv`          => `env::var` or `env::var_string`
* `os::getenv_as_bytes` => `env::var`
* `os::setenv`          => `env::set_var`
* `os::unsetenv`        => `env::remove_var`

Many function signatures have also been tweaked for various purposes, but the
main changes were:

* `Vec`-returning APIs now all return iterators instead
* All APIs are now centered around `OsString` instead of `Vec<u8>` or `String`.
  There is currently on convenience API, `env::var_string`, which can be used to
  get the value of an environment variable as a unicode `String`.

All old APIs are `#[deprecated]` in-place and will remain for some time to allow
for migrations. The semantics of the APIs have been tweaked slightly with regard
to dealing with invalid unicode (panic instead of replacement).

The new `std::env` module is all contained within the `env` feature, so crates
must add the following to access the new APIs:

    #![feature(env)]

[breaking-change]

1 files changed, 53 insertions, 0 deletions

diff --git a/src/libstd/sys/unix/c.rs b/src/libstd/sys/unix/c.rs
index 9016d1a2c99..7a17e8b6f1e 100644
--- a/src/libstd/sys/unix/c.rs
+++ b/src/libstd/sys/unix/c.rs
@@ -66,6 +66,53 @@ pub const MSG_DONTWAIT: libc::c_int = 0x40;
 
 pub const WNOHANG: libc::c_int = 1;
 
+#[cfg(target_os = "linux")]
+pub const _SC_GETPW_R_SIZE_MAX: libc::c_int = 70;
+#[cfg(any(target_os = "macos",
+          target_os = "freebsd"))]
+pub const _SC_GETPW_R_SIZE_MAX: libc::c_int = 71;
+#[cfg(target_os = "android")]
+pub const _SC_GETPW_R_SIZE_MAX: libc::c_int = 0x0048;
+
+#[repr(C)]
+#[cfg(target_os = "linux")]
+pub struct passwd {
+    pub pw_name: *mut libc::c_char,
+    pub pw_passwd: *mut libc::c_char,
+    pub pw_uid: libc::uid_t,
+    pub pw_gid: libc::gid_t,
+    pub pw_gecos: *mut libc::c_char,
+    pub pw_dir: *mut libc::c_char,
+    pub pw_shell: *mut libc::c_char,
+}
+
+#[repr(C)]
+#[cfg(any(target_os = "macos",
+          target_os = "freebsd"))]
+pub struct passwd {
+    pub pw_name: *mut libc::c_char,
+    pub pw_passwd: *mut libc::c_char,
+    pub pw_uid: libc::uid_t,
+    pub pw_gid: libc::gid_t,
+    pub pw_change: libc::time_t,
+    pub pw_class: *mut libc::c_char,
+    pub pw_gecos: *mut libc::c_char,
+    pub pw_dir: *mut libc::c_char,
+    pub pw_shell: *mut libc::c_char,
+    pub pw_expire: libc::time_t,
+}
+
+#[repr(C)]
+#[cfg(target_os = "android")]
+pub struct passwd {
+    pub pw_name: *mut libc::c_char,
+    pub pw_passwd: *mut libc::c_char,
+    pub pw_uid: libc::uid_t,
+    pub pw_gid: libc::gid_t,
+    pub pw_dir: *mut libc::c_char,
+    pub pw_shell: *mut libc::c_char,
+}
+
 extern {
     pub fn gettimeofday(timeval: *mut libc::timeval,
                         tzp: *mut libc::c_void) -> libc::c_int;
@@ -92,6 +139,12 @@ extern {
     pub fn sigaddset(set: *mut sigset_t, signum: libc::c_int) -> libc::c_int;
     pub fn sigdelset(set: *mut sigset_t, signum: libc::c_int) -> libc::c_int;
     pub fn sigemptyset(set: *mut sigset_t) -> libc::c_int;
+
+    pub fn getpwuid_r(uid: libc::uid_t,
+                      pwd: *mut passwd,
+                      buf: *mut libc::c_char,
+                      buflen: libc::size_t,
+                      result: *mut *mut passwd) -> libc::c_int;
 }
 
 #[cfg(any(target_os = "macos", target_os = "ios"))]