By default define CTFCONVERT and CTFMERGE to be no-ops in sys.mk.

Don't test for /usr/bin/ctfconvert or /usr/bin/ctfmerge -- that is
completely bonkers and breaks the build if the host's ctfconvert and
ctfmerge are too old.

Strip the broken conditionals in the .c.o &c. rules.

bsd.own.mk will override CTFCONVERT and CTFMERGE with TOOL_CTFCONVERT
and TOOL_CTFMERGE if MKCTF=yes.

XXX The way CTFCONVERT and CTFMERGE are integrated into the rules is
bogus -- it is not safe to interrupt them.  We need some intermediate
object to do this right.

Please let me know if I broke anything with this -- I am not an elvin
or dwarven wizard, nor a dtrace expert.  But this has been breaking
builds for ages now and nobody else seems to want to do anything
about it.

(riastradh)