add no-op POSIX_SPAWN_USEVFORK to spawn.h

the bit is reserved anyway for ABI-compat reasons; this documents it
and makes it so we can have posix_spawnattr_setflags check for flag
validity without hard-coding an anonymous bit value.

include/spawn.h

@@ -21,6 +21,7 @@ struct sched_param;
 #define POSIX_SPAWN_SETSIGMASK 8
 #define POSIX_SPAWN_SETSCHEDPARAM 16
 #define POSIX_SPAWN_SETSCHEDULER 32
+#define POSIX_SPAWN_USEVFORK 64
 #define POSIX_SPAWN_SETSID 128
 
 typedef struct {