Merge #1761

1761: Add non-standard Linux `SysconfVar` variants r=asomers a=stevenengler

Closes #1240.

You can find these options near the end of https://man7.org/linux/man-pages/man3/sysconf.3.html.

I can see that the libc crate defines these for Linux and Android, but I'm not sure if they're defined for any others as well.

Co-authored-by: Steven Engler <opara@cs.georgetown.edu>

CHANGELOG.md

@@ -6,6 +6,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 ## [Unreleased] - ReleaseDate
 ### Added
 
+- Added four non-standard Linux `SysconfVar` variants
+  (#[1761](https://github.com/nix-rust/nix/pull/1761))
 - Added const constructors for `TimeSpec` and `TimeVal`
   (#[1760](https://github.com/nix-rust/nix/pull/1760))
 - Added `aio_writev` and `aio_readv`.

src/unistd.rs

@@ -2668,6 +2668,19 @@ pub enum SysconfVar {
     /// Integer value indicating version of the X/Open Portability Guide to
     /// which the implementation conforms.
     _XOPEN_VERSION = libc::_SC_XOPEN_VERSION,
+    /// The number of pages of physical memory. Note that it is possible for
+    /// the product of this value to overflow.
+    #[cfg(any(target_os="android", target_os="linux"))]
+    _PHYS_PAGES = libc::_SC_PHYS_PAGES,
+    /// The number of currently available pages of physical memory.
+    #[cfg(any(target_os="android", target_os="linux"))]
+    _AVPHYS_PAGES = libc::_SC_AVPHYS_PAGES,
+    /// The number of processors configured.
+    #[cfg(any(target_os="android", target_os="linux"))]
+    _NPROCESSORS_CONF = libc::_SC_NPROCESSORS_CONF,
+    /// The number of processors currently online (available).
+    #[cfg(any(target_os="android", target_os="linux"))]
+    _NPROCESSORS_ONLN = libc::_SC_NPROCESSORS_ONLN,
 }
 
 /// Get configurable system variables (see