Allow breakpoint commands to be set from Python

Allow breakpoint commands to be set from Python

This changes the Python API so that breakpoint commands can be set by
writing to the "commands" attribute.

ChangeLog
2018-05-04  Tom Tromey  <tom@tromey.com>

	PR python/22731:
	* NEWS: Mention that breakpoint commands are writable.
	* python/py-breakpoint.c (bppy_set_commands): New function.
	(breakpoint_object_getset) <"commands">: Use it.

doc/ChangeLog
2018-05-04  Tom Tromey  <tom@tromey.com>

	PR python/22731:
	* python.texi (Breakpoints In Python): Mention that "commands" is
	writable.

testsuite/ChangeLog
2018-05-04  Tom Tromey  <tom@tromey.com>

	PR python/22731:
	* gdb.python/py-breakpoint.exp: Test setting breakpoint commands.