Bridge code is limited to 256 ports per bridge because the Spanning
Tree Protocol has limit of one octet for port number.  This code
fixes:
	* bogus unlock in error path when port list is full.
	* passes different error status for out of memory, vs
	  port list full.
	* O(n) vs O(n^2) lookup for free port number
	* since port and priority are both limited to one byte
	  don't store them as int's
	* makes limit explicit in code