chap-secrets: add pool name support

[themiron]

Chap-secrets' ipdb uses 4th field as static peer ipv4 address. With no radius and multiple same username sessions, it's impossible to use non-default pool for such sessions. Abuse chap-secret's 4th field as pool=name to specify session's pool name. With ippool module loaded after chap-secrets (default order), it will be used for allocation from the specified poll name.

Compatibility considerations:

• pppd will skip 'pool=*' with warn 'unknown host in auth. address list' same as 5th field - shaper, because starting from 4th field pppd parse list of value. so, no new effects here.
• previous versions of accel-ppp will parse 'pool=*' as invalid 255.255.255.255 address.
• with no 'pool=*' in chap-secrets or with no chap-secrets loaded, no behavior change.
• with no ippool loaded, session will get no peer address.
• with ippool loaded before chap-secrets, chap-secrets's ipdb will not be used, therefore neither ip addess not pool name will has no effect.
• if chap-secrets' pool is invalid or not found, default pool will be used by ippool or address came from radius.
• chap-secret's pool name might override pool came from radius, if radius module is loaded after chap-secrets and no address came from radius.

[themiron]

or, please advise, what would be other preferable way to use pass dp->pool for allocation

[xebd]

maybe just check if 4th field is valid ip4 address if not consider it as pool name

[themiron]

done, 4th field isn't empty and doesn't start with reserved chars (*-!), assume it as pool name