options: change license of most files to LGPL (except options.c/.h)

All authors of the current code have agreed (as far as this commit
requires).

options.c/options.h will take more effort, because it contains all the
option declarations, and thus is touched extremely often.

m_option.c is technically still GPL, because of commit 2c82d5a1d85378dd0
(michael has agreed to LGPL, but only once the core of mpv is LGPL).

The geometry parsing code in m_option.c was originally by someone who
could not be reached. However, it was heavily rewritten anyway, and only
the syntax remains (i.e. not copyright-relevant).

parse_commandline.c contains a change by "adland" (commit 1d0ac71ae8ba),
who could not be reached - this this specific part is GPL only.
Fortunately, it matters only for DVD (and even then is more like a hack,
but whatever).

There are some other relevant changes, but they have all been reverted,
moved somewhere else, deleted, or replaced.