Introduce timer_* support

This commit adds support for the signal timer mechanism in POSIX, the
mirror to timerfd on Linux.

Resolves #1424

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>

CHANGELOG.md

@@ -25,6 +25,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
   (#[1547](https://github.com/nix-rust/nix/pull/1547))
 - Added getter methods to `MqAttr` struct
   (#[1619](https://github.com/nix-rust/nix/pull/1619))
+- Added `timer` support
+  (#[1622](https://github.com/nix-rust/nix/pull/1622))
 
 ### Changed
 ### Fixed

Cargo.toml

@@ -70,7 +70,7 @@ sched = ["process"]
 signal = ["process"]
 socket = []
 term = []
-time = []
+time = ["signal"]
 ucontext = ["signal"]
 uio = []
 users = ["features"]

src/sys/mod.rs

@@ -201,3 +201,13 @@ feature! {
     #[allow(missing_docs)]
     pub mod timerfd;
 }
+
+#[cfg(any(target_os = "freebsd",
+          target_os = "illumos",
+          target_os = "linux",
+          target_os = "netbsd"
+))]
+feature! {
+    #![feature = "time"]
+    pub mod timer;
+}