bgpd: add missing "partial" flag checks (BZ#676)

bgpd: add missing "partial" flag checks (BZ#676)

ORIGIN handling function used to have "partial" bit check and recent
commits added it for NEXT_HOP, MULTI_EXIT_DISC and ATOMIC_AGGREGATE
cases. This commit adds "partial" check for AS_PATH and LOCAL_PREF
cases, which should leave attributes 1 through 6 inclusive completely
covered with attribute flags checks.

* bgp_attr.c
  * bgp_attr_origin(): use bit-by-bit checks for better diagnostics
  * bgp_attr_aspath(): add flag check
  * bgp_attr_local_pref(): idem