virtual_oss(8): Fix cuse.ko check

[fernape]

Summary

virtual_oss(8) checks if cuse(3) is loaded. However, kldload(2) ends up calling
kern_kldload that checks permissions first. It is only later on in
linker_load_module that -EEXIST is returned if the module is already loaded.

That means that users that can't load modules, always get a -EPERM error even if
cuse.ko is already loaded and ready to use.

$ kldstat | grep cuse
 2    1 0xffffffff8233a000     c148 cuse.ko
$ virtual_oss
 virtual_oss: Failed to load cuse kernel module: Operation not permitted
$

Change it to check if the kernel module is already loaded and try load it if it
isn't.

In addition move the program's arguments parsing early on because otherwise, a
user can't even access the program's help if cuse.ko is not loaded and the
user doesn't have permissions to do it.

Test Plan

As a regular user with no permissions to load modules run:

virtual_oss -h

An Operation not permitted is shown.

Load cuse(3) as root:

sudo kldload cuse

Run:
virtual_oss
virtual_oss: Failed to load cuse kernel module: Operation not permitted

But the module is loaded and should be used.