Add waitid and related constants and types.

waitid() is a variation on waitpid() with a marginally more
convenient way of reporting the status, and a couple of handy
additional features, such as the ability to peek at an exit
status without consuming it.  It's in POSIX.1-2008 and should
be available on all supported Unixes.

Along with it come the type 'idtype_t' and the constants
WEXITED, WSTOPPED, WCONTINUED, and WNOWAIT.  Theconstants
were alre dy defined for unix/notbsd platforms.

Patch incomplete: several targets are going to have to add
definitions of siginfo_t, but I'm not sure which ones yet.

src/unix/bsd/mod.rs

@@ -271,6 +271,10 @@ pub const NOFLSH: ::tcflag_t = 0x80000000;
 
 pub const WNOHANG: ::c_int = 0x00000001;
 pub const WUNTRACED: ::c_int = 0x00000002;
+pub const WEXITED: ::c_int = 0x00000004;
+pub const WSTOPPED: ::c_int = 0x00000008;
+pub const WCONTINUED: ::c_int = 0x00000010;
+pub const WNOWAIT: ::c_int = 0x00000020;
 
 pub const RTLD_NOW: ::c_int = 0x2;
 pub const RTLD_DEFAULT: *mut ::c_void = -2isize as *mut ::c_void;

src/unix/mod.rs

@@ -16,6 +16,13 @@ pub type cc_t = ::c_uchar;
 pub enum DIR {}
 pub enum locale_t {}
 
+#[repr(C)]
+pub enum idtype_t {
+    P_ALL = 0,
+    P_PID = 1,
+    P_PGID = 2,
+}
+
 s! {
     pub struct group {
         pub gr_name: *mut ::c_char,
@@ -447,6 +454,8 @@ extern {
                link_name = "waitpid$UNIX2003")]
     pub fn waitpid(pid: pid_t, status: *mut ::c_int, options: ::c_int)
                    -> pid_t;
+    pub fn waitid(idtype: idtype_t, id: id_t, infop: *mut ::siginfo_t,
+                  options: ::c_int) -> ::c_int;
     #[cfg_attr(all(target_os = "macos", target_arch = "x86"),
                link_name = "write$UNIX2003")]
     pub fn write(fd: ::c_int, buf: *const ::c_void, count: ::size_t)