More moves of string functions out of lib/lib.h.

More moves of string functions out of lib/lib.h.

match_pattern* has been renamed to str_match_pattern*,
(word|sep)split has been renamed to str_(word|sep)split,
all of them have been moved to lib/string.h.

md5_to_hex() and hex_to_md5() have been moved to lib/string.h,
but MD5_SIZE and MD5_HEX_SIZE to lib/md5.h. The conversion
functions will be replaced by a generic hexdumper soon.