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 spec

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.

(cherry picked from commit f555b39e6bb7cbfbe1905e90f64c4dfc4456fabb)

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