Add back 802.1D Spanning Tree Protocol (STP) library code.

Add back 802.1D Spanning Tree Protocol (STP) library code.

At one point, the OVS distribution contained an IEEE 802.1D Spanning
Tree Protocol (STP) library written by Ben Pfaff and based on the
802.1D-1998 reference code.  It was never integrated into ovs-vswitchd,
so it was removed as part of commit ba18611 (Remove vestigial support
for Spanning Tree Protocol.)

This commit reintroduces the library, cleans up a few spots, and makes
it build cleanly against new code.  A future commit will have
ovs-vswitchd use this library.