Disable forceful setting of '-j1'

Disable forceful setting of '-j1', and allow setting WORLDJOBS
and KERNJOBS in the config.  This also allows setting the configurations
FREEBSD_BUILDWORLD_{,EXTRA}_ARGS and FREEBSD_BUILDKERNEL_{,EXTRA}_ARGS
to include '-jN', which is passed through to the world/kernel builds.

Tested against head@r261419 with '-j10' for world, and '-j6' for kernel,
without any ill side effects.

Sponsored by:	The FreeBSD Foundation