ipoe: send unclassified packet info to userspace via netlink

* implemented per CTRL mppe config
* implemented vendor/attr options to specify which radius attribute containes ip pool name
* implemented adaptive lcp echo functionality
* implemented leaf qdisc attachment (only sfq at present)
* implemented Challenge attribute handling (l2tp)
* implemented unit(interface) cache
* fixed ip-pre-up script result handling
* fixed time range start detection on startup/config reload
* fixed reading of req-limit option (radius)
* fixed unexpected radattr file removal

     If enabled accel-pppd will not destroy interface immediately after corresponding session is terminated, instead interface will be brought down  and placed to cache for later use for new sessions.
     It should reduce kernel interface creation/deletion rate lack and increase responsibility of daemon

Low interface creation/deletion rate is kernel issue and should be fixed in 3.5.

This reverts commit 9ae4a015.

Revert "fdtrash: don't queue descriptors after context is closed (fix segfault when stopping accel-pppd)"

This reverts commit 2e568932.

ppp: if signle-session=replace down first session's interface before second comes up (should fix unexpected ospf routes deletion)

 to ignore them, instead of refusing connection.
Signed-off-by: NDenys Fedoryshchenko <denys@visp.net.lb>

pppd_compat: use EV_PPP_PRE_FINISHED event to start ip-down script (fixes unexpected radattr file removal)