libusb: add libusb_log_level enum

This is documented on http://libusb.sourceforge.net/api-1.0/group__lib.html
Sync ucom.4, uplcom.4 and uslcom.4 with FreeBSD.
devfs - sync devfs after cloning a device

 * Use case are devices such as tty_pty which create a ptm/foo node as
   the cloned one, but also create a slave node (in pts/foo).

 * Without this, it's possible that ptm/foo is created before pts/foo
   is, and the program may race and not see pts/foo yet.
Implemented casuword for vkernel64. Fix two typos in casuword for pc64.
Add a ufoma(4) manual page (from FreeBSD).
usb.4: Sync parts of the manual page with FreeBSD.
Merge branch 'master' into compat_linux