SNMPv3 patch

Hi all

The attached patch fixes some bugs in the parser of snmphosts.cfg that make
the SNMPv3 implementation mostly useless. These are all off-by-one errors
that cause strings like "passphrase=" to never be found in the
configuration file.

I've also changed two instances of debug logging to error logging because
they're errors that need operator/sysadmin intervention: one is an error in
the configuration (eg "Host X ... no such name") and the other is a
connection timeout.

Cheers
Jeremy