configure: Use AC_ARG_WITH also for openssl options.

Currently the check for openssl is performed by CHECK_SSL() in
configure.ac. This function searches the given directory for the
openssl-header but does not check the existence and usability of
the openssl libraries. The argument of --enable-ssldir, if given,
is tried first, and each member of a hard-coded list of directories
is searched next.

This patch replaces CHECK_SSL() by checks similar to those for
other headers and libraries. In particular, we now also check
for the openssl libraries, and the configure options are now
called --with-openssl-headers and --with-openssl-libs rather than
--enable-ssldir.