Auto merge of #2983 - HarveyHunt:posix-spawn-setsid, r=JohnTitor

linux: Add POSIX_SPAWN_SETSID flag

This flag allows the child process created by POSIX spawn to create a new session and become leader of a new process group. Expose the flag so that Rust code can use it.

Author: bors

libc-test/semver/linux.txt

@@ -1696,6 +1696,7 @@ POSIX_SPAWN_SETSCHEDULER
 POSIX_SPAWN_SETSIGDEF
 POSIX_SPAWN_SETSIGMASK
 POSIX_SPAWN_USEVFORK
+POSIX_SPAWN_SETSID
 PROT_GROWSDOWN
 PROT_GROWSUP
 PR_CAPBSET_DROP

src/unix/linux_like/linux/mod.rs

@@ -1593,6 +1593,7 @@ pub const POSIX_MADV_RANDOM: ::c_int = 1;
 pub const POSIX_MADV_SEQUENTIAL: ::c_int = 2;
 pub const POSIX_MADV_WILLNEED: ::c_int = 3;
 pub const POSIX_SPAWN_USEVFORK: ::c_int = 64;
+pub const POSIX_SPAWN_SETSID: ::c_int = 128;
 
 pub const S_IEXEC: mode_t = 64;
 pub const S_IWRITE: mode_t = 128;