Log Message:
syncthing-gtk: Overhaul SUBST of /usr paths

This had a SUBST block to fix /usr/share to ${PREFIX}/share.  It was
doing this to files where the pattern didn't match, and also missed
fixin up /usr/bin.  Split into a bin and a share SUBST, and loosen the
regexp (specifically, to match /syncthing rather than just
/syncthing-gtk).  Tested to basically work on NetBSD 8.  Desktop files
etc. appear much better than before.

(Found by newish SUBST noop check.)