Move mkdir_recursive to common/filestuff.c

Move mkdir_recursive to common/filestuff.c

This moves mkdir_recursive from dwarf-index-cache.c to
common/filestuff.c, and also changes it to return a boolean that says
whether or not it worked.

gdb/ChangeLog
2018-10-27  Tom Tromey  <tom@tromey.com>

	* unittests/mkdir-recursive-selftests.c: New file.
	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
	unittests/mkdir-recursive-selftests.c.
	* dwarf-index-cache.c (mkdir_recursive): Move to
	common/filestuff.c.
	(index_cache::store): Check return value of mkdir_recursive.
	(create_dir_and_check, test_mkdir_recursive): Move to new file.
	(_initialize_index_cache): Don't register test.
	* common/filestuff.h (mkdir_recursive): Declare.
	* common/filestuff.c (mkdir_recursive): Move from
	dwarf-index-cache.c.  Return bool.