licences: Introduce Zandronum

This licence only covers Zandronum-specific code, but over the years,
through ZDooM, GZDooM, Skulltag, and others, this has a borrowed a whole
bunch of code licenced under a whole bunch of non-GPL compliant
licences, etc.
Including all of them here, or simply referencing them here wouldn't
make much sense, especially since they are not included in the sources.
licences: Introduce BUILDLIC

This is the licence for the Build engine by Ken Silverman, from which
ZDooM and Zandronum borrow code.
licences: Introduce DUMB-0.9.3

This licence covers some of the sound code in Zandronum.
licences: Introduce MUSLIB

This is the licence for MUSlib, from which code was borrowed in
Zandronum.
licences: Introduce PHDL

This is the Paul Hsieh derivative license, also containing the text from
the Paul Hsieh exposition license.
According to ZDoom's wiki: "The hash function ZDoom uses for storing and
retrieving names, found in c_dispatch.cpp, is based on Paul Hsieh's
SuperFastHash algorithm."
Introduce games-fps/zandronum
repository_mask: Mask scm version of zandronum