bgpd: Fixes for recent well-known-attr check patch.

bgpd: Fixes for recent well-known-attr check patch.

* bgp_attr.c: Recent patch to tighten well-known attr checks and apply that
  to all AFIs has some breakage with MP-extensions and GR, which needs to be
  fixed.
  (bgp_attr_check) Graceful Restart EoR can be an empty UPDATE for IPv4/uni.
  MP-Ext allow UPDATE with just MP_UNREACH_NLRI. Check for these and return
  proceed.
  NEXT_HOP becomes optional, if MP_REACH_NLRI is present and there's no
  v4 NLTI, update NEXT_HOP check accordingly.
  Print the missing attr in string form in the log message.
  (bgp_attr_parse) AS_PATH need not be there, so
  bgp_attr_munge_as4_attrs call needs to be conditional on that.

(cherry picked from commit aed1b556cf2f55680ae09d7ad1a1f22729dea8c5)

Conflicts:
	bgpd/bgp_attr.c