Return unique_xmalloc_ptr from gdbscm_scm_to_string

Return unique_xmalloc_ptr from gdbscm_scm_to_string

This changes gdbscm_scm_to_string to return a unique_xmalloc_ptr and
then fixes all the callers.  This allows for the removal of some
cleanups.

gdb/ChangeLog
2018-07-17  Tom Tromey  <tom@tromey.com>

	* guile/scm-param.c (pascm_set_func, pascm_show_func)
	(compute_enum_list, pascm_set_param_value_x)
	(gdbscm_parameter_value): Update.
	* guile/guile-internal.h (gdbscm_scm_to_string): Update.
	(gdbscm_scm_to_host_string): Update.
	* guile/scm-math.c (vlscm_convert_typed_value_from_scheme):
	Update.
	* guile/scm-cmd.c (cmdscm_add_completion): Update.
	* guile/scm-pretty-print.c (ppscm_print_string_repr): Update.
	* guile/scm-string.c (gdbscm_scm_to_string): Return
	unique_xmalloc_ptr.
	(gdbscm_scm_to_host_string): Likewise.