bsdiff: if file is /dev/fd/*, don't open/close it. Just use the fd.

The updater, in an attempt to keep bspatch from open/closing the raw
block device frequently, passes a file descriptor and a path of
/dev/fd/(some number) as the path. This patch causes bspatch to
recognize that magic file path and not call open/close.

This dramatically reduces resource usage on client machines performing
delta updates.

BUG=7636
TEST=Installed delta on device

Review URL: http://codereview.chromium.org/3698005