meson: Make dlopen=all the default

meson: Make dlopen=all the default

As of earlier commit, all dependencies can be dlopen'd on demand. Do
that for all optional libraries by default, but still allow it to be
overridden during build.

Example numbers/benefits taken from `modprobe --show-depends nouveau`:

 - fewer instr - 50% reduction, 1.2M -> 0.5M
 - less memory use - Rss by ~33%, 3M -> 2M
 - reduced attack surface

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/420
Signed-off-by: Lucas De Marchi <demarchi@kernel.org>