Eliminate remaining gdb/guile cleanups

Eliminate remaining gdb/guile cleanups

The remaining gdb/guile cleanups all handle the memory returned by
gdbscm_scm_to_c_string.

This commit makes gdbscm_scm_to_c_string return a
gdb::unique_xmalloc_ptr instead of a naked pointer, and eliminates the
remaining cleanups.

gdb/ChangeLog:
2018-07-19  Pedro Alves  <palves@redhat.com>

	* guile/guile-internal.h (gdbscm_scm_to_c_string): Now returns a
	gdb::unique_xmalloc_ptr.
	* guile/scm-breakpoint.c (gdbscm_set_breakpoint_condition_x):
	Adjust to use dbscm_wrap and gdb::unique_xmalloc_ptr.
	* guile/scm-exception.c (gdbscm_exception_message_to_string): Use
	copy-initialization.
	* guile/scm-pretty-print.c (ppscm_print_children): Use
	gdb::unique_xmalloc_ptr instead of cleanups.
	(gdbscm_apply_val_pretty_printer): Remove cleanups.
	* guile/scm-string.c (gdbscm_scm_to_c_string): Now returns a
	gdb::unique_xmalloc_ptr.
	* guile/scm-type.c (gdbscm_type_field, gdbscm_type_has_field_p):
	Adjust to use gdb::unique_xmalloc_ptr.
	* guile/scm-utils.c (extract_arg): Adjust.
	* guile/scm-value.c (gdbscm_value_field): Adjust to use
	gdb::unique_xmalloc_ptr instead of a cleanup.