build: Increase warning level to 2

As discussed in !357, we should increase the number of
enabled compiler warnings to catch a lot more silent bugs.  Increase the
meson-global warning level to 2, but excluding the `unused-parameter`
and `old-style-declaration` warnings.  Additionally, disable warnings
for libraries as we cannot fix that code anyway.

Signed-off-by: Rahix <rahix@rahix.de>