MAC/do: Rules: <from> and <to> parts now to be separated by '>'

Previously, we would accept only ':' as the separator, which makes
parsing of the rule specification harder for humans, especially those
people that are used to UNIX systems where ':' is used as the separator
in PATH. With ':', the <from> and <to> parts can look like two
different elements that are unrelated, especially to these eyes.

Change parse_single_rule() so that '>' is also accepted as a separator
between <from> and <to>, and promote it as the one to use. During
a transition period, we will still allow the use of ':' for backwards
compatibility.

The manual page update comes from separate revision D49628. ':' has
been completely removed from it on purpose.

Reviewed by: bapt, manpages (ziaee)
MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49627

(cherry picked from commit f01d26dec67fb6597438ed765269b85d1099a6fa)