Introduce startmsg and use it in rc scripts

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