Remove some gotos from Python

This patch slightly refactors a couple of spots in the Python code to
avoid some gotos.

gdb/ChangeLog
2017-02-10  Tom Tromey  <tom@tromey.com>

	* python/python.c (do_start_initialization): New function, from
	_initialize_python.
	(_initialize_python): Call do_start_initialization.
	* python/py-linetable.c (ltpy_iternext): Use explicit returns, not
	goto.