Some xerox_mfp scanners, such as Samsung SCX-4521F, hangs when
their OUT ENDPOINT receives CLEAR HALT command, which is performed
in usb open and close functions of the backend.
The issue seem to be dependant on USB controller and assumingly
happens due to some sort of incompatibility of USB implementation
in the scanner.

Skip sending this command by default and add
SANE_XEROX_USB_HALT_WORKAROUND env variable to execute
CLEAR HALT commands.