libcompat_minix-centric cleanup

libcompat_minix-centric cleanup

remove some old minix-userland-specific stuff

	. /etc/ttytab as a file, and minix-compat function (fftyslot()),
	  replaced by /etc/ttys and new libc functions
	. also remove minix-specific nlist(), cuserid(), fttyslot(), v8 regex
	  functions and <compat/regex.h>
	. and remaining minix-only utilities that use them
	. also unused <compat/pwd.h> and <compat/syslog.h> and
	  redundant <sys/sigcontext.h>