Add a 'realclean' target, which simply and quickly removes everything that was built in previous 'make buildworld' and 'make buildkernel'.  The target knows enough to run a 'chflags -R 0' and a second 'rm' if the first 'rm' ran into any errors while removing files.
Add a 'realclean' target, which simply and quickly removes everything
that was built in previous 'make buildworld' and 'make buildkernel'.  The
target knows enough to run a 'chflags -R 0' and a second 'rm' if the first
'rm' ran into any errors while removing files.

Suggested by:	email with Richard Coleman Re: upcoming 64b-time_t changes.
Reviewed by:	imp, marcel, and others on -hackers
MFC after:	1 week

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