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.
diff --git a/include/spawn.h b/include/spawn.h
index f3e9e23..bba57ce 100644
--- a/include/spawn.h
+++ b/include/spawn.h
@@ -21,6 +21,7 @@
#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 {