tools: Move shared OCaml code to mllib/ directory.

Previously a lot of shared code lived in the resize/ directory for no
particular reason.

This is just code motion.
uri: Fix two compiler warnings.

We have to include the right header so that guestfs___free_string_list
is declared.  Unfortunately that means ensuring -I src is passed to
the compiler in every tool subdirectory.

Also fix (bogus) compiler warning about incorrect type of the
parameter to caml_copy_string_array.