Some updates to tristar

First commit renames `$(PREFIX)` in Makefile.in to `$(PREFIX_DIR)` and reinstates some `$(DESTDIR)`s.

Second commit has the bin-install.pl script create files for {ufo,panda,p6doc}-{p,j,m} as running the normal file using the postfix'ed perl6. Since that explanation doesn't make sense: Instead of relying on, e.g., ufo to provide ufo-j, when it already has logic to deal with whatever executable is running it, just copy `ufo` to `bin/` as usual and create a new file `ufo-j` that runs something like `perl6-j ufo`, but with absolute paths.

Third commit updates MIME::Base64 for @retupmoca++.