Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Rewrite node to support multiple hooks, alike to ng_l2tp, to use one pair
of pptpgre and ksocket nodes for all calls between two peers. This patch
modifies node's API by adding new "session_%04x" hook names support, while
keeping backward compatibility.

Together with appropriate user-level support (by latest mpd5) it gives
huge performance benefits for case of multiple active calls between
two peers because of avoiding data duplication and extra socket processing.
On my benchmarks I have got more then 10 times speedup for the 200
simultaneous PPTP calls between two peers.
In conclusion, it allows now to build effective "clients <=> PAC <=> PNS"
setups.

/* -> /*- for license, minor formatting changes

Add 'enableWindowing' configuration knob to the ng_pptpgre(4) netgraph node.

Submitted by: Michael Bretterklieber <mbretter@a-quadrat.at>
MFC after: 2 weeks

Use a single style of multiple inclusion protection for Netgraph headers.

Reviewed by: archie, harti, emax

Fix GCC warnings caused by initializing a zero length array. In the process,
simply things a bit by getting rid of 'struct ng_parse_struct_info' which
was useless because it only contained one field.

MFC after: 2 weeks

Since neither archie nor I work at Whistle any more, change our email
addresses to be the more usefu @freebsd.org ones
so we can keep getting bug-reports.
- man pages to follow..

Take advantage of the new unsigned and hex integer types.

Several fixes:
- Fix slowness when operating over fast connections, where the timeout(9)
granularity is on the same order of magnitude as the round trip time.
timeout(9) can happen up to 1 tick early, which was causing receive
ack timeouts to happen too early, causing bogus "lost" packets.
- Increase the local time counter to 64 bits to avoid roll-over.
- Keep statistics on memory allocation failures.
- Add a new option to always include the ack when sending data packets.
Might be useful in high packet loss situations. Might not.

Fix a couple of bugs:
- Properly handle 32 bit sequence numbers when they wrap around
- Don't drop GRE packets with stale ACK numbers, just ignore the ACK
- Close race between node being shutdown and timer going off
Also add support for lots of statistics, and control message ASCIIfication

New netgraph node type 'pptpgre': this performs GRE encapsulation
for the PPTP protocol as specified in RFC 2637.