(gdba): Remove gdb-quit (previously removed) from

(gdba): Remove gdb-quit (previously removed) from
documentation.
(gdb-source, gdb-source-info): Update to assembler unnecessary
as its done after each GDB command anyway.
(gdb-pre-prompt): Use with-current-buffer.
(gdb-insert-field): Add help-echo text.
(gdb-invalidate-assembler): Re-display of assembler now done in
gdb-info-breakpoints-custom.
(gdb-info-breakpoints-custom): Force re-display of assembler to
happen *after* update of breakpoints buffer.
(gdb-display-source-buffer): Don't choke if gdb-source-window
isn't visible.
(gdb-put-string, gdb-put-arrow): Remove free variables.