snmp: link with Net-SNMP later

Some systems contain static libraries in the output of `net-snmp-config
--agent-libs`. It is not possible to build a convenience library with a
static library. Therefore, we just link with the appropriate libs in the
last step. This should fix #57.
redhat: enable SNMP support on RHEL 5 and CentOS 5 by default

This is related with the fix in the previous commit (ae7f4c2).
coverity: fix memory leak in lldpcli

CID 1198257 (#1 of 1): Resource leak (RESOURCE_LEAK)
coverity: fix memory leak in lldpd when the same option is repeated
coverity: fix a false positive in SNMP transport

`remote` is not expected to be a NULL-terminated string. However, just
in case it is used as one somewhere, we alloc the appropriate space for
it.
coverity: change a bit the annotations

We want coverity to ignore those errors. It doesn't seem to be the case.
Merge branch 'master' into coverity_scan