l2tp: add Vendor-Name attribute to SCCRP

* implemented named IP pools (radius Framed-Pool attribute)
* implemented logging to system logger (log_syslog)
* implemented ability to send cli commands via snmp
* implemented PADI limiting (pppoe)
* implemented simultaneous radius requests limiting
* various bug fixes and improvements

This reverts commit ee41cba6.

This reverts commit 2405fd6b.

* implemented SNMP support (using net-snmp AgentX)
* implemented multiple radius servers support
* implemented IPv6:
  - PPP IPV6CP protocol
  - Neighbor Discovery protocol in part of Router Advertisement
  - DHCPv6 server including Prefix Delegation extension
  - ipv6-pool module
  - Radius Framed-Interface-Id, Framed-IPv6-Prefix, Delegated-IPv6-Prefix attributes
* fixed MPPE handling
* implemented noauth mode (login without authentication)
* improved CCP/IPCP handling
* included builtin crypto library (optional)
* improved l2tp handling
* fixed builtin shaper temporary rate change functionality
* various bug fixes and improvements

ppp: drop route of previous session before bringing up new if single-session=replace (make ospf happy)