bgpd: fix handling of AS path data

bgpd: fix handling of AS path data

* bgpd/bgp_aspath.c
  * assegments_parse(): add handling of AS4_PATH input, update bounds
    checks, add check for AS segment type
  * aspath_parse(): add handling of AS4_PATH input, expect
    assegments_parse() to do length checking
  * aspath_empty(): update for the new function prototype
* bgpd/bgp_aspath.h: ditto
* tests/aspath_test.c: ditto
* bgpd/bgp_attr.c
  * bgp_attr_aspath(): add handling of AS4_PATH input, update flags
    checks, change returned type
  * bgp_attr_as4_path(): discard, superseded by bgp_attr_aspath()
  * bgp_attr_parse(): update respectively