Significantly faster concurrent buildworld times

[Matthew Dillon]

    I did a pass on the buildworld infrastructure and added a new
    features to allow SUBDIR recursions to run concurrently.  This
    should improve buildworld -j 12 (or similar) significantly.  I
    was able to get a 28% improvement on our quad-core (8 thread) Xeons
    (1075 -> 769 seconds).

    This is still a bit experimental in that there may be build dependencies
    that we haven't ferreted out yet.  In particular, you might have to
    update your 'install' program to the latest in master to avoid a race
    inside its mkdir() function which could error-out the build (only if you
    are doing make -j N on your buildworlds).

    This work cleaned up probably 70-80% of the bottlenecks we had in the
    buildworld.  There are far fewer periods showing idle cpu during the
    build with these changes.

    http://gitweb.dragonflybsd.org/dragonfly.git/commit/d2e9c9d8664f753a0d599eceed1dd98ffa7ef479
    http://gitweb.dragonflybsd.org/dragonfly.git/commit/67be553814c6242d4a801d26dc2f6e5ca4b1aa8a
    http://gitweb.dragonflybsd.org/dragonfly.git/commit/0e6b9ee838cf6370c2c8e3ea723839c383de96cc
    http://gitweb.dragonflybsd.org/dragonfly.git/commit/6e73105ec5492ebba66b83ced8a62e16f87e0498

					-Matt
					Matthew Dillon 
					<dillon at backplane.com>