replace it with the absolute path of .PARSEDIR, so that sub-makes
launched from objdirs (eg kernel) can still find the correct mk files.
Reviewed by: obrien
src/sys and the rest of the tree for builds.
o eliminate including bsd.mkopts.mk for the moment in kern.opts.mk
o No need to include src.opts.mk at all anymore. The reasons for it
are now coverted in sys.mk and src.sys.mk.