util: New function ovs_scan().

util: New function ovs_scan().

This new function is essentially an implementation of sscanf() with
slightly different behavior (see the comment) that is more convenient for
Open vSwitch internal use.  Also, this implementation ought to work out of
the box on Windows, which has a defective sscanf() that lacks the 'hh'
modifier required to scan into a char variable.

Signed-off-by: Ben Pfaff <blp@nicira.com>