Add basic Python API for convenience variables

Add basic Python API for convenience variables

This adds a basic Python API for accessing convenience variables.
With this, convenience variables can be read and set from Python.
Although gdb supports convenience variables whose value changes at
each call, this is not exposed to Python; it could be, but I think
it's just as good to write a convenience function in this situation.

This is PR python/23080.

Tested on x86-64 Fedora 26.

2018-04-22  Tom Tromey  <tom@tromey.com>

	PR python/23080:
	* NEWS: Update for new functions.
	* python/py-value.c (gdbpy_set_convenience_variable)
	(gdbpy_convenience_variable): New functions.
	* python/python-internal.h (gdbpy_convenience_variable)
	(gdbpy_set_convenience_variable): Declare.
	* python/python.c (python_GdbMethods): Add convenience_variable,
	set_convenience_variable.

doc/ChangeLog
2018-04-22  Tom Tromey  <tom@tromey.com>

	PR python/23080:
	* python.texi (Basic Python): Document gdb.convenience_variable,
	gdb.set_convenience_variable.

testsuite/ChangeLog
2018-04-22  Tom Tromey  <tom@tromey.com>

	PR python/23080:
	* gdb.python/python.exp: Add convenience variable tests.