stand: Consolidate checking for boot flags driven by environment vars

e.g. boot_mute, boot_single, boot_verbose, and friends; we checked for these
in multiple places, consolidate into common/ and allow a setting of "NO" for
any of these to turn them off. This allows systems with multiple
loader.conf(5) or loader.conf(5) overlay systems to easily turn off
variables in later processed files by setting it to NO.

Reported by: Nick Wolff @ iXsystems
Reviewed by: imp