ipsec: T1210: split out pool from remote-access configuration

Remote access IP pools can now be defined at a global level and referenced
in IPSec remote-access connections. To defined a pool use:

set vpn ipsec remote-access pool global-ipv4 name-server '172.16.1.1'
set vpn ipsec remote-access pool global-ipv4 prefix '192.168.0.0/24'
set vpn ipsec remote-access pool global-ipv6 name-server '2001:db8::1'
set vpn ipsec remote-access pool global-ipv6 prefix '2001:db8:1000::/64'

A connection can then reference the pool:

set vpn ipsec remote-access connection foo pool 'global-ipv4'
set vpn ipsec remote-access connection foo pool 'global-ipv6'