eval-when-compile a few defvars to quieten the byte-compiler.

eval-when-compile a few defvars to quieten the byte-compiler.
(f90-xemacs-flag): Wrap in eval-and-compile.
(f90-font-lock-keywords): Doc fix.
(f90-mode-abbrev-table): Use the 6th (system-flag) argument of
define-abbrev if available, but work without it as well.
(f90-mark-subprogram-overlay): Variable deleted.
(f90-mark-subprogram): No need to be silent about push-mark.
Get rid of the silly overlay bit.
(f90-abbrev-start): unread-command-event is obsolete in XEmacs too.