Cleanup calls to Fgarbage_collect.

Cleanup calls to Fgarbage_collect.
* lisp.h (maybe_gc): New prototype.
(consing_since_gc, gc_relative_threshold, memory_full_cons_threshold):
Remove declarations.
* alloc.c (maybe_gc): New function.
(consing_since_gc, gc_relative_threshold, memory_full_cons_threshold):
Make them static.
* bytecode.c (MAYBE_GC): Use maybe_gc.
* eval.c (eval_sub, Ffuncall): Likewise.
* keyboard.c (read_char): Likewise.  Adjust call to maybe_gc
to avoid dependency from auto-save feature.