vboxheadless: Fix start of specific machine after base r291770.
The vboxheadless rc script has its own pid checking, so the rc.subr
version in run_rc_command is not needed.
PR: 212074
In the rc.d scripts, change assignments to rcvar to use the
literal name_enable wherever possible, and ${name}_enable
when it's not, to prepare for the demise of set_rcvar().
In cases where I had to hand-edit unusual instances also
modify formatting slightly to be more uniform (and in
some cases, correct). This includes adding some $FreeBSD$
tags, and most importantly moving rcvar= to right after
name= so it's clear that one is derived from the other.