sbin/shutdown: Support time units as in 'shutdown -r +5sec'

Units supported: s, sec, m, min, h, hour.

Differential Revision:	https://reviews.freebsd.org/D1272


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275818 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f