Drop dep. on mod_perl in zembperl.load, enable mod_perl in postinst if present

This should still enable mod_perl by default if libembperl-perl +
Recommends are installed, but should also works if
libapache2-mod-perl2 is not installed (as it's only recommended).

While I believe that this should work and fix #810655, I'm currently
unable to test it in Sid or Stretch. So please verify that it's
working before uploading.