Rework eln hash filename strategy

Rework eln hash filename strategy

Generate eln filename hashing also the source file content in the form:

/absolute/path/filename.el + content ->
eln-cache/filename-path_hash-content_hash.eln

	* src/lread.c (maybe_swap_for_eln): Always call
	Fcomp_el_to_eln_filename on an existing source file.

	* src/comp.c (md5.h, sysstdio.h, zlib.h): New include.
	(comp_hash_string): Use md5 instead of sha512.
	(MD5_BLOCKSIZE): New macro.
	(accumulate_and_process_md5, final_process_md5, md5_gz_stream)
	(comp_hash_source_file): New functions.
	(Fcomp_el_to_eln_filename): Rework for hasing using also source
	file content.

	* src/lread.c (maybe_swap_for_eln): Rename el_name -> src_name as
	this can be also a have .el.gz extention.