man: reset OPTIND before parsing args

man: reset OPTIND before parsing args

From jilles: POSIX requires that a script set `OPTIND=1` before using
different sets of parameters with `getopts`, or the results will be
unspecified.

The specific problem observed here is that we would execute `man -f` or
`man -k` without cleaning up state from man_parse_args()' `getopts`
loop.  FreeBSD's /bin/sh seems to reset OPTIND to 1 after we hit the
second getopts loop, rendering the following shift harmless; other
/bin/sh implementations will leave it at what we came into the loop at
(e.g., bash as /bin/sh), shifting off any keywords that we had.

Input from:	jilles
Reviewed by:	allanjude, bapt, imp
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D32063