accept(2) may and can return EAGAIN, document it.

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272884 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Do not add late flag when file= is specified because it has a bad
side-effect.  The specified file should exist before the fstab line.

Reported by:	wblock (long time ago)
MFC after:	1 day


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272885 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f