Incorporate a parallelization option for snapshot/release builds
when the 'parallel' option is set to 'parallel', which changes the
invoked build_release() function to parallelbuild_release().

Note, this has significant impact on the machine, most notably load
averages of 220+ when the -CURRENT builds fire off simultanously.

This is toggleable in main.conf, as it could likely set machines on
fire.

In my initial tests, build times were reduced as follows:

• 12-CURRENT: ~22 hours -> ~4 hours
• 11-STABLE: ~10 hours -> ~3 hours
• 10-STABLE: ~5 hours -> ~2 hours [1]

While here, unset WITH_VMIMAGES when re-sourcing the build config
file, so it does not pollute arm builds, discovered by accident.

Sponsored by: The FreeBSD Foundation