Stop copying the licence text into C source code.

Stop copying the licence text into C source code.

Now all the uses of the licence text or the short copyright notice get
it from a new header "licence.h", which in turn is built by a Perl
script licence.pl invoked by mkfiles.pl, using LICENCE itself as the
source.

Hence, I can completely remove a whole section from the list of
licence locations in CHECKLST.txt :-)

(cherry picked from commit 9ddd071ec28050b3be572f25f3ae7d44e46e4039)

Conflicts:
	unix/gtkdlg.c
	windows/winpgnt.c

(cherry-picker's notes: one conflict was just changed context, the
other was deleting a copy of the licence that wasn't quite the same
between branches)