Refactor window-system configuration

Refactor window-system configuration

This change streamlines the window system selection code in
configure.in and moves many common function declarations from
window-specific headers to frame.h.  It introduces a new TERM_HEADER
macro in config.h: we set this macro to the right header to use for
the window system for which we're compiling Emacs and have source
files include it indirectly.  This way, we don't have to teach every
file about every window system.