Moved src/microcode/prgtkio.c to src/gtk/gtkio.c.

Moved src/microcode/prgtkio.c to src/gtk/gtkio.c.

This simplifies the configuration of src/microcode/, removing
conditional references to the Gtks.

* doc/gtk/gtk.texinfo: Fixed the "make clean" suggestion to the proper
"make distclean".  Simplified the build instructions, then made them
more novice-friendly, avoiding cautions about writable /usr/local/bin,
etc.

* src/configure.ac: Moved the --with-gtk support back from
microcode/achost.ac, where it is no longer needed.

* src/gtk/.gitignore: Added gtkio.c, a product of gtkio.c.stay.

* src/gtk/Clean.sh: Removed prgtkio.so; added gtkio.c.

* src/gtk/Makefile-fragment: Added gtkio.c, which is not compiled like
a regular shim.  It uses the machine's OS interface, and narrowly
avoids including scheme.h.

* src/gtk/gtk-shim.h: Added extern declarations for functions that
used to be Gtk primitives.

* src/gtk/gtk.cdecl: Declare the new functions.

* src/gtk/gtk.sf: There is no prgtkio module, nor gtkio primitives
anymore.

* src/gtk/gtkio.c.stay: This is src/microcode/prgtkio.c with the
  primitives turned into simple C functions.

* src/gtk/: main.scm, thread.scm: Replace Gtk primitives with callouts
to their equivalent C functions.

* src/microcode/: configure.ac, achost.ac: Punted --with-gtk.

* src/microcode/makegen/: Makefile.in.in, files-optional.scm: Punted
prgtkio.

* src/microcode/prgtkio.c: Moved to src/gtk/gtkio.c.stay.

* src/microcode/: pruxffi.c, pruxffi.h: Added abort_to_c(), so gtkio.c
  does not need prim.h, scheme.h and everything.