Add add_internal_function overload

Add add_internal_function overload

add_internal_function sets a command destroyer that frees the doc
string.  However, many callers do not pass in an allocated doc string.

This adds a new overload to clearly differentiate the two cases,
fixing the latent bug.

gdb/ChangeLog
2019-11-26  Tom Tromey  <tom@tromey.com>

	* value.h (add_internal_function): Add new overload.  Move
	documentation from value.h.
	* value.c (do_add_internal_function): New function.
	(add_internal_function): Use it.  Add new overload.
	(function_destroyer): Don't free doc.
	* python/py-function.c (fnpy_init): Update.

Change-Id: I3f6df925bc6b3e1bccbad9eeebc487b908bb5a2a