(mempcpy): Don't define if a system header defines it.

(mempcpy): Don't define if a system header defines it.
Don't include stdio.h, stdlib.h, unistd.h, strdup.h.
(longest_relative_suffix): New function.
(path_concat): Use it.  Assume first argument is not NULL.
Port to DOS.  Omit redundant separators.
Report an error instead of returning NULL.
Use mempcpy instead of memcpy.
(xpath_concat): Remove: not declared or used.