bgpd: handle ICMP type and code from flowspec

bgpd: handle ICMP type and code from flowspec

It is possible for flowspec entries containing ICMP rule to insert PBR
entries based on ICMP type and ICMP code.
Flowspec ICMP filtering can either have icmp type or icmp code or both.
Not all combinations are permitted:
- if icmp code is provided, then it is not possible to derive the
  correct icmp value. This will not be installed
- range of ICMP is authorised or list of ICMP, but not both.
- on receiving a list of ICMPtype/code, each ICMP type is attempted to
  be associated to ICMP code. If not found, then ICMPtype is combined
  with all known ICMP code values associated to that ICMP type.
- if a specific ICMP type/code is needed, despite the ICMP code/type
  combination does not exist, then it is possible to do it by forging a
  FS ICMP type/code specific for that.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>