strptime: %s format fix.

Almost never needed in real life because %s is tends to be
only one format spec.
1) Return code of gmtime_r() is checked.
2) All flags are set.

Submitted by:	ache
MFC after:	3 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272441 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f