Update lang/logo from 4.5 to 5.1
added to knobs to the port:
* EMACS_LOGO_MODE to build and install the emacs logo mode -> requires emacs to
* WITH_FULL_DOCS to build and install ps, pdf and html documentation ->
requires teTeX and ghostscript to build.
If WITH_FULL_DOCS is defined, NOPORTDOCS will be undefined!
Respect NOPORTDOCS otherwise, i.e. the plain text documentation will be
not installed, if NOPORTDOCS is set.