Make gtkmain.c contain the actual main().

Make gtkmain.c contain the actual main().

Instead of main() living in uxputty.c and uxpterm.c, and doing a
little bit of setup before calling the larger pt_main() in gtkmain.c,
I've now turned things backwards: the big function in gtkmain.c *is*
main(), and the small pieces of preliminary setup in uxputty.c and
uxpterm.c are now a function called setup() which is called from
there. This will allow me to reuse the rest of ux{putty,pterm}.c, i.e.
the assorted top-level bits and pieces that distinguish PuTTY from
pterm, in the upcoming OS X application that will have its own main().