MFC r257824,257826-257830,258411: Updates to sysrc(8)

MFC r257824,257826-257830,258411: Updates to sysrc(8)
257824: Fix a bug with `-d' form working as documented
257826: Add `--version' long option
257827: Add a `-c' option for `check only'
257828: Comments and whitespace
257829: Fix a bug with `-e' introduced by above 257828
257830: Document SYSRC_VERBOSE enviroment variable in the manual
258411: Revert the above 257830 (both merged to get .Dd bump in man-page)

git-svn-id: svn://svn.freebsd.org/base/stable/10@260679 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f