0.6.12.21:

0.6.12.21:
	(realized I was confused about DEFAULT-DIRECTORY ('cause it's
		in SB-INT, not SB-INT) so it doesn't need to be
		marked deprecated after all. As I get older, perhaps I
		should work on smaller systems..)
	deleted unused %SET-DEFAULT-DIRECTORY stuff
	deleted related Unix chdir() stuff
	rewrote UNIX-CURRENT-DIRECTORY to use getcwd() and do its
		own error-handling; got rid of DEFAULT-DIRECTORY
	deleted related Unix getwd() stuff
	restored SIZE-T (but this time through grovel_headers.c)
		in order to interface to getcwd() more cleanly
	deleted commented-out clutter in ldso-stubs.S
	renamed UNIX-CURRENT-DIRECTORY to POSIX-GETCWD, and made
		POSIX-GETCWD/ variant to do the old CMU-CL-style
		trailing #\/ thing