Return gdbpy_ref from some Python string functions

This changes python_string_to_unicode,
python_string_to_target_python_string, and
host_string_to_python_string to return gdbpy_ref.

gdb/ChangeLog
2018-11-04  Tom Tromey  <tom@tromey.com>

	* python/python.c (gdbpy_parameter_value): Update.
	* python/python-internal.h (python_string_to_unicode)
	(python_string_to_target_python_string)
	(host_string_to_python_string): Return gdbpy_ref.
	* python/py-utils.c (python_string_to_unicode)
	(unicode_to_encoded_python_string)
	(unicode_to_target_python_string)
	(python_string_to_target_string)
	(python_string_to_target_python_string): Return gdbpy_ref.
	(python_string_to_host_string): Update.
	(host_string_to_python_string): Return gdbpy_ref.
	* python/py-symtab.c (stpy_get_filename, stpy_get_producer)
	(stpy_fullname): Update.
	* python/py-progspace.c (pspy_get_filename, pspy_solib_name):
	Update.
	* python/py-prettyprint.c (print_string_repr): Update.
	* python/py-objfile.c (objfpy_get_filename, objfpy_get_username)
	(objfpy_get_build_id): Update.
	* python/py-breakpoint.c (bppy_get_location)
	(bppy_get_expression, bppy_get_condition, bppy_get_commands):
	Update.