boot/loader: Handle the /boot/modules.local

boot/loader: Handle the /boot/modules.local

Weather /boot/modules.local is used during boot or by kldload(8)
is controlled by a new "local_modules" loader variable.

If local_modules is set to "YES", /boot/modules.local is also
appended to kern.module_path sysctl variable for use by kldload(8).
Setting variable to any other value disables that.

By default allow modules to searched in /boot/modules.local directory
since it does not shadow the default kernel modules and has lower
directory search priority.