input: change license to LGPL

cehoyos adds the step_property command in 7a71da01d, and it could be
argued that copyright of this still applies to the later add/cycle
commands (a668ae0ff90c4). While I'm not sure if this is really the case,
stay conservative for now and mark these commands as GPL-only. Mark the
command.c code too, although that is not being relicensed yet.

I'm leaving the MP_CMD_* enum items, as they are obviously different.

In commit 116ca0c7682, "veal" (essentially an anonymous author) adds an
"osd_show_property_text" command (well, the commit message says "based
on" that person's code, so it's not clear how much is from him or from
albeu, who agreed to LGPL). This was later merged again with the
"osd_show_text" command, and then all original code was removed in
commit 58cc0f637f, so I claim that no copyright applies anymore. (Though
technically the input.conf addition still might be copyrighted, so I'm
just dropping it to get rid of the thought.)

"kiriuja" added 2f376d1b39 (sub_load etc.) and be54f4813 (switch_audio).
The latter is gone. I would argue that the former is fully rewritten
with commits b7052b431c9 and 0f155921b0. But like in the step_property
case, I will be overly conservative for now, and mark them as GPL-only,
as this is potentially shaky and should be thought through first. (Not
bothering with the command define/enum in the header, as it will be
unused in LGPL mode anyway.)

keycodes.c/h can be GPL, except for commit 2b1f95dcc2f8, which is a
patch by someone who wasn't asked yet. Before doing something radical, I
will wait for a reply.

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