[SV 61621] Don't use POSIX_SPAWN_RESETIDS with posix_spawn()

[SV 61621] Don't use POSIX_SPAWN_RESETIDS with posix_spawn()

When make is invoked in a environment where the user namespace is
restricted, such as under unshare(1) (on GNU/Linux), it won't be able
to find its real UID so the effective UID can't be set to it and
posix_spawn() will fail with EINVAL.

It's not less safe to run recipe jobs using the same UID values that
make was invoked with, so don't worry about this flag.

* src/job.c (child_execute_job): Don't set POSIX_SPAWN_RESETIDS flag.