libenc: Fixed some extraneous bytes in zx_ENC_WO_any_elem().
libenc: Rewrite of the default case of zx_LEN_WO_any_elem().

As the old code had bugs where the length differed from the one
generated by zx_ENC_WO_any_elem(), and the code flow wasn't
identical, locating the bug was needlessly hard.

The code flow is now similar to the one in ZX_ENC_WO_any_elem(),
and a few bugs that were found along the way fixed.
Makefile: Gentoo has apr in /usr/include/apr-1/.