lib, bgp: add initial support for asdot format

lib, bgp: add initial support for asdot format

AS number can be defined as an unsigned long number, or
two uint16 values separated by a period (.). The possible
valus are:
- usual 32 bit values : [1;2^32 -1]
- <1.65535>.<0.65535> for dot notation
- <0.65535>.<0.65535> for dot+ notation.

The 0.0 value is forbidden when configuring BGP instances
or peer configurations.

A new ASN type is added for parsing in the vty.
The following commands use that new identifier:
- router bgp ..
- bgp confederation ..
- neighbor <> remote-as <>
- neighbor <> local-as <>
- clear ip bgp <>
- route-map / set as-path <>

An asn library is available in lib/ and provides some
services:
- convert an as string into an as number.
- parse an as path list string and extract a number.
- convert an as number into a string.

Also, the bgp tests forge an as_zero_path, and to do that,
an API to relax the possibility to have a 0 as value is
specifically called from the tests.

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