Add more free licenses and disambiguate packages

[psvenk]

Closes #21. Closes #22. This pull request adds many free licenses and variants of already included licenses and also disambiguates many packages with a custom license field. Additionally, a few licenses are reorganized (see below for explanation).

CCPL:by-nc-sa is deliberately excluded (despite its inclusion in #22) from the list of free licenses because it is considered nonfree by both the FSF and Debian (and the OSI only evaluates software licenses).

GPLv2.1 is excluded because (to my knowledge) it is not a real license and the issue comes from the software author's or the package maintainer's end (maybe LGPL version 2.1 was intended?)

RDL is excluded because I could not find what license it refers to (it is not in SPDX) and what package(s) use it.

Notes on licenses that are not clearly variants of already listed ones:

GPL3-only: The Arch wiki states that GPL3 refers to software licensed under GPL version 3 or later; so I added this designation to denote software licensed under GPL version 3 only (e.g. parts of cups-filters).

SIL Open Font License 1.1 and Bitstream Vera License: Used by ttf-hack.

CUSTOM: This is a variant of custom and therefore ambiguous.

custom:OpenMPI: a modified 3-clause BSD license used by openmpi. Free according to DFSG (included in Debian main).

custom:GFL: a variant of LPPL v1.3c or later with an additional non-legally-binding clause, used by otf-latin-modern and oft-latinmodern-math.

custom:Apache 2.0 with LLVM Exception: Apache 2.0 with these exceptions, used by llvm-libs; the misspelled version is used by lib32-llvm-libs.

custom:libpng: A permissive (non-copyleft) license used by libpng and libpng12 that meets DFSG.

custom:GD: Following the convention of Debian.

custom:HPND: Historical Permission Notice and Disclaimer (OSI-approved).

custom:bitstream-vera: License of Bitstream Vera; meets DFSG.

custom:libxcomposite: This is the MIT-style license used by libxcomposite; meets DFSG.

custom:TekHVC: Used by parts of libx11; meets DFSG.

custom:Qhull: Permissive license used by qhull; meets DFSG.

custom:Info-ZIP: Permissive license used by zip; meets DFSG.

custom:X11-DEC: Used by parts of xorg-xkbutils (THE INFORMATION IN THIS SOFTWARE...); meets DFSG.

custom:lsof: License of lsof; meets DFSG.

custom:Sleepycat: An OSI-approved copyleft license used by db.

custom:PUEL: This does not seem to be a free software license. Section 2 of the PUEL infringes on freedom 0 by limiting the purposes for which the software can be use; the rest of the document may infringe on other freedoms as well. In addition, Debian has placed its package virtualbox-guest-additions-iso in the non-free section.

custom:Khronos and custom:SGI: Licenses used by parts of mesa which meet DFSG.

custom:w3m: License of w3m; packaged in Debian main.

custom:tcl: License of tcl; packaged in Debian main.

custom:bzip2: License of bzip2; packaged in Debian main.

custom:unlicense/custom:Unlicense: Public domain dedication approved by the FSF.

custom:neovim: Neovim is licensed under a mixture of Apache 2.0 and the Vim license, both of which are free licenses already included here.

[psvenk]

I forgot to add: I also included a change so that if a lib32 package has an ambiguous license and the non-lib32 version of that package is listed in AMBIGUOUS_PACKAGES, the license(s) for that package are used.