Introduce startmsg and use it in rc scripts

startmsg is a new rc.subr(8) function function to be used instead of
echo(1) when for boot messages. It replaces the often forgotten

check_startmsgs && echo ...

with

startmsg ...

No functional change intended.

I adjusted the commit message and did some final clean-ups of the patch
before committing.

PR: 255207
Reported by: Jose Luis Duran <jlduran@gmail.com>
Reviewed by: imp, 0mp
Approved by: imp (src)
Differential Revision: https://reviews.freebsd.org/D34514

(cherry picked from commit 325ebf37d8efc6488754051fcc2b1aaa40cefd8b)