(slurp_file): New function.

(slurp_file): New function.
(xbm_image_p): Handle case of in-memory XBM files.
(xbm_scan): Rewritten to work on memory buffers instead of files.
(xbm_read_bitmap_data): Renamed from xbm_read_bitmap_file_data.
Work on memory buffers instead of files.  If DATA is null test
if buffer looks like an in-memory XBM file.
(xbm_load_image): Renamed from xbm_load_image_file.  Work on
memory buffers instead of files.
(xbm_file_p): New function.
(xbm_load): Accept :data DATA where DATA is an in-memory XBM file.