Auto merge of #2509 - devnexen:linux_posix_spawn_usevfork, r=Amanieu

linux/android add POSIX_SPAWN_USEVFORK (mostly no-op now) extension.

closes #2269

Author: bors

libc-test/semver/linux.txt

@@ -1529,6 +1529,7 @@ POSIX_SPAWN_SETSCHEDPARAM
 POSIX_SPAWN_SETSCHEDULER
 POSIX_SPAWN_SETSIGDEF
 POSIX_SPAWN_SETSIGMASK
+POSIX_SPAWN_USEVFORK
 PROT_GROWSDOWN
 PROT_GROWSUP
 PR_CAPBSET_DROP

src/unix/linux_like/linux/mod.rs

@@ -1330,6 +1330,7 @@ pub const POSIX_MADV_NORMAL: ::c_int = 0;
 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 S_IEXEC: mode_t = 64;
 pub const S_IWRITE: mode_t = 128;