Add address keyword to Value.format_string

This makes it possible to disable the address in the result string:

const char *str = "alpha";

(gdb) py print(gdb.parse_and_eval("str").format_string())
0x404000 "alpha"
(gdb) py print(gdb.parse_and_eval("str").format_string(address=False))
"alpha"

gdb/ChangeLog:

2020-12-18  Hannes Domani  <ssbssa@yahoo.de>

	* python/py-value.c (valpy_format_string): Implement address keyword.

gdb/doc/ChangeLog:

2020-12-18  Hannes Domani  <ssbssa@yahoo.de>

	* python.texi (Values From Inferior): Document the address keyword.

gdb/testsuite/ChangeLog:

2020-12-18  Hannes Domani  <ssbssa@yahoo.de>

	* gdb.python/py-format-string.exp: Add tests for address keyword.