gdb: extension languages finish_initialization to initialize

gdb: extension languages finish_initialization to initialize

Now that both Python and Guile are fully initialized from their
respective finish_initialization methods, the "finish" in the method
name doesn't really make sense; initialization starts _and_ finishes
with that method.

As such, this commit renames finish_initialization to just initialize.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* extension-priv.h (struct extension_language_ops): Rename
	'finish_initialization' to 'initialize'.
	* extension.c (finish_ext_lang_initialization): Renamed to...
	(ext_lang_initialization): ...this, update comment, and updated
	the calls to reflect the change in struct extension_language_ops.
	* extension.h (finish_ext_lang_initialization): Renamed to...
	(ext_lang_initialization): ...this.
	* guile/guile.c (gdbscm_finish_initialization): Renamed to...
	(gdbscm_initialize): ...this, update comment at definition.
	(guile_extension_ops): Update.
	* main.c (captured_main_1): Update call to
	finish_ext_lang_initialization.
	* python/python.c (gdbpy_finish_initialization): Rename to...
	(gdbpy_initialize): ...this, update comment at definition, and
	update call to do_finish_initialization.
	(python_extension_ops): Update.
	(do_finish_initialization): Rename to...
	(do_initialize): ...this, and update comment.