Don't rely on FindBin module to locate utils.pm

As the FindBin module doesn't work with ePN, set the path to utils.pm
explicitly at build time.  Keep using FindBin additionally, so that the
plugins can also be executed from the build directory.

Closes #1271.
plugins/netutils.h: Adjust UNIX_PATH_MAX for BSD

On at least FreeBSD, NetBSD, OpenBSD, DragonFly, and OS X, this is
hard-coded at 104 bytes.

Closes #1267.
NEWS: Update version number

We're going to release version 2.0, not 1.6.
NEWS: Add a warning regarding check_snmp

Tell users that check_snmp might now return CRITICAL in cases where it
used to return OK, and how to deal with that.