Rename make_symbol_completion_list_fn -> symbol_completer

Rename make_symbol_completion_list_fn -> symbol_completer

"make_symbol_completion_list_fn" is odly named when you look at a list
of "standard" completers, like the Python/Guile completer lists
adjusted by this patch.  Rename / move it to completers.h/c, for
consistency.

gdb/ChangeLog:
2017-07-17  Pedro Alves  <palves@redhat.com>

	* completer.c (symbol_completer): New function, based on
	make_symbol_completion_list_fn.
	* completer.h (symbol_completer): New declaration.
	* guile/scm-cmd.c (cmdscm_completers): Adjust.
	* python/py-cmd.c (completers): Adjust.
	* symtab.c (make_symbol_completion_list_fn): Delete.
	* symtab.h (make_symbol_completion_list_fn): Delete.
	* cli/cli-decode.c (add_cmd): Adjust.