The header scrcpy.h is intended to be the "public" API. It should not
depend on other internal headers.
Therefore, declare all required structs in this header and adapt
internal code.
The verbosity was set either to info (in release mode) or debug (in
debug mode).
Add a command-line argument to change it, so that users can enable debug
logs using the release:
scrcpy -Vdebug
The platform-specific code for net.c was implemented in sys/*/net.c.
But the differences are quite limited, so use ifdef-blocks in the single
net.c instead.
