fix Arista telnet prompt regexp
use inet_ntop, inet_ntop instead of self-made ones

self-made format_ip* and parse_ip* were leaved for compatibility with
pre-5.1 PHPs and PHPs built without IPv6 support. All API calls have not
changed.

is_inet_avail: checks inet_ntop and inet_pton for existance and IPv6
support
ip4_format: try to use inet_ntop, inet_pton before self-made code
ip6_format: idem
ip_parse: idem
ip4_parse: idem
ip6_parse: idem
retry fetching running-config from Cisco IOS devs

some devices (e.g. Cisco IOS) refuse to print running configuration
while they are busy. The best way of treating this is retry a few times
before failing the request.

ERetryNeeded: new exception class
ios12ReadVLANConfig: throwing ERetryNeeded if busy error message seen.
ios12SpotConfigText: idem.
queryDevice: loop queryTerminal 3 times if ERetryNeeded is catched
eos4BuildSwitchport: improve DFL VLAN detection