rc.subr: Introduce startmsg

[jlduran_gmail.com]

Summary

A simple function to be used instead of echo(1) when starting, specifically, it replaces the --often forgotten-- stanza check_startmsgs && echo on rc files.

No functional change intended.

Suggested in PR 255207.

[0mp]

libexec/rc/rc.subr

724

This is missing $@, right?

[jlduran_gmail.com]

Add missing $@.

[jlduran_gmail.com]

libexec/rc/rc.subr

724

I'll submit some changes later, as this won't work.

[jlduran_gmail.com]

Sorry, for a moment I doubted about changing echo -n to printf.

[jlduran_gmail.com]

Briefly mention the new function in rc.subr(8).

[0mp]

share/man/man8/rc.subr.8

946

I'll drop echo(1) from the final commit. It does not really help to have his reference here.

[jlduran_gmail.com]

Landed as 325ebf37d8efc6488754051fcc2b1aaa40cefd8b.

Thank you very much!