This removes PKG_VERSION and PKG_RELEASE from most Makefiles, as the
value was never useful for Gluon packages; instead, PKG_VERSION is set
to 1 in gluon.mk.

It also removes two other weird definitions:

- gluon-iptables-clamp-mss-to-pmtu replicating the old PKG_VERSION logic
  from gluon-core, but without the fixed PKG_BUILD_DIR to prevent
  unnessary rebuilds
- gluon-hoodselector set GLUON_VERSION=3

Calling git describe directly breaks isolation between the build system
and packages. Replace this with proper .config variables, like we
already do for GLUON_RELEASE.

Also replace the PKG_VERSION hack with a static '1', as we do for other
packages - while having those version numbers in opkg was cute, it was
also entirely useless. Having a fixed PKG_VERSION allows us to remove
the PKG_BUILD_DIR override as well.

We already have a proper message when the creation of an alias fails
because of a name conflict. Also add a message when the primary filename
of a domain config is already occupied by another domain's alias.

Also add an 'Error:' prefix to the existing message to easier to see.

The old bash-based parsing code was way too complex. Replace it with Lua.

When two domains alias the same name (or one aliases another), display a
meaningful error message like:

    Failed to alias domain 'foo' as 'bar', name already taken by
    domain 'baz'.

Solves a recursive dependency problem.

While we're at it, also fix the description string.

Based-on-patch-by: lemoer <git@irrelefant.net>

The domain configs are not checked yet, and not used for anything.

Based-on-patch-by: lemoer <git@irrelefant.net>

This will allow us to use its content from other languages than Lua as
well.

PKG_FILE_DEPENDS caused the whole site repo to be hashed to determine if
gluon-site needed rebuilding. While this is normally no problem,
alternative build setups sometimes put the Gluon repo inside the site repo,
causing long build times and parallel build failures.

Having two POT files will only cause confusion.