Allow EPROTOTYPE for SOCK_CLOEXEC but unsupported by socket/socketpair

Allow EPROTOTYPE for SOCK_CLOEXEC but unsupported by socket/socketpair

If SOCK_CLOEXEC is defined (usually because accept4 is implemented),
check for EPROTOTYPE (the POSIX errno for invalid socket types) in
addition to EINVAL as errno indicating whether socket and socketpair
do not support SOCK_CLOEXEC (and other SOCK_* flags).

[adapted by Chengwei Yang to give _dbus_connect_exec() the same treatment]
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=69073
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>