Introduce and use make_unique_xstrdup

Introduce and use make_unique_xstrdup

Adds an utility function to make it shorter to write the common case
of wrapping an xstrdup with a unique_xmalloc_ptr, and uses it
throughout.

Note: I tried to put this in common/common-utils.h near skip_spaces,
etc. but that is included in common/common-defs.h before
common/gdb_unique_ptr.h is included, so it would fail to compile
because gdb::unique_xmalloc_ptr isn't defined at that point yet.  I
tried moving the gdb_unique_ptr.h inclusion before common-utils.h, but
that doesn't work because gdb_unique_ptr.h depends on common-utils.h
for xfree.

gdb/ChangeLog:
2019-06-04  Pedro Alves  <palves@redhat.com>

	* common/gdb_unique_ptr.h (make_unique_xstrdup): New.

	* ada-lang.c (catch_ada_completer): Use make_unique_xstrdup.
	* breakpoint.c (condition_completer): Likewise.
	* cli/cli-dump.c (scan_expression): Likewise.
	* common/filestuff.c (mkdir_recursive): Likewise.
	* common/gdb_tilde_expand.c (gdb_tilde_expand_up)
	* common/pathstuff.c (gdb_realpath, gdb_realpath_keepfile)
	(gdb_abspath): Likewise.
	* compile/compile-cplus-types.c
	(compile_cplus_instance::decl_name): Likewise.
	* completer.c (complete_explicit_location):
	(signal_completer, reg_or_group_completer_1): Likewise.
	* cp-support.c (cp_remove_params_if_any): Likewise.
	* fbsd-tdep.c (fbsd_core_vnode_path): Likewise.
	* guile/scm-safe-call.c (gdbscm_safe_eval_string): Likewise.
	* infcmd.c (strip_bg_char): Likewise.
	* linespec.c (copy_token_string): Likewise.
	* mi/mi-main.c (output_cores): Likewise.
	* psymtab.c (psymtab_search_name):
	* symfile.c (test_set_ext_lang_command): Likewise.
	* target.c (target_fileio_read_stralloc): Likewise.
	* tui/tui-regs.c (tui_reggroup_completer): Likewise.
	* value.c (complete_internalvar): Likewise.

gdb/gdbserver/ChangeLog:
2019-06-04  Pedro Alves  <palves@redhat.com>

	* server.c (captured_main): Use make_unique_xstrdup.