more control key formatting

more control key formatting

The previous fix to use highc(UNCTRL(x)) worked for ^A through ^Z,
but not for NUL (yielded ^` instead of usual ^@) or ^[ through ^_
(yielded lowercase ^{ and so on).  The problem was UNCTRL(); it
shouldn't have been forcing on the lowercase bit to begin with.
Also, the code that used UNMETA() for formatting wouldn't work as
intended for M-control char since it stripped off the 8th bit but
didn't apply any fixup for control chars.

Just get rid of ISCTRL/ISMETA/UNCTRL/UNMETA and use the existing
visctrl() routine instead.  (Its 3.6.0 edition didn't handle
M-control char, but the to-be-3.6.1 branch has done so since a
week or two after the 3.6.0 release.)