net-snmp: run snmp in "special" thread

NET-SNMP is built on top of stale file descriptor multiplexing mechanism (select)
which prevents to use descriptors with number >= 1024.
Due to accel-ppp uses a lots of descriptors NET-SNMP easy overflows this limit and fails
and even may crash whole accel-ppp process.
This patch runs snmp in special thread which does not share file descritor table
with main process, so it can use less descriptors and live in __FD_SETSIZE limit.

Signed-off-by: Dmitry Kozlov <xeb@mail.ru>
Revert "net-snmp: run snmp in "special" thread"

This reverts commit f2774e5a02d29de71eb64a64565b23090b6f6c5e.

snmp_thread() needs to be run as regular thread to allow Valgrind
analysis. Using a different namespace for file desciptors will be
done later on using unshare().

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
net-snmp: unshare file descriptors namespace

Use unshare(CLONE_FILES) to create a local file descriptors namespace
for the SNMP thread. This is similar to what was done in bf5340
'net-snmp: run snmp in "special" thread', but without calling clone()
directly. So the net-snmp plugin keeps running in a regular pthread.

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
1.7.4 release
* fixed various bugs
* introduced accel-cmd program
* ippool: impelented persistance
* radius: implemented default-realm option
* radius: proper handling of Framed-IP-Address=255.255.255.254
* shaper: time range and other fixes
* shaper: implemented mtu option
* net-snmp: fixed run out of FD_SETSIZE
Merge remote-tracking branch 'remotes/upstream/1.7' into upstream-1.7