Fix "set" handling of Python parameters

Fix "set" handling of Python parameters

It's long bothered me that setting a Python parameter from the CLI
will print the "set" help text by default.  I think usually "set"
commands should be silent.  And, while you can modify this behavior a
bit by providing a "get_set_string" method, if this method returns an
empty string, a blank line will be printed.

This patch removes the "help" behavior and changes the get_set_string
behavior to avoid printing a blank line.  The code has a comment about
preserving API behavior, but I don't think this is truly important;
and in any case the workaround -- implementing get_set_string -- is
trivial.

Regression tested on x86-64 Fedora 26.

2018-04-26  Tom Tromey  <tom@tromey.com>

	* NEWS: Mention new "set" behavior.
	* python/py-param.c (get_set_value): Don't print an empty string.
	Don't call get_doc_string.

gdb/doc/ChangeLog
2018-04-26  Tom Tromey  <tom@tromey.com>

	* python.texi (Parameters In Python): Update get_set_string
	documentation.