MFC r295385: semget(): Check for [EEXIST] error first.

MFC r295385: semget(): Check for [EEXIST] error first.

Although POSIX literally permits failing with [EINVAL] if IPC_CREAT and
IPC_EXCL were both passed, the semaphore set already exists and has fewer
semaphores than nsems, this does not allow an application to retry safely:
if the [EINVAL] is actually because of the semmsl limit, an infinite loop
would result.

PR:		206927

git-svn-id: svn://svn.freebsd.org/base/stable/10@297747 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f