Fix uses of ProtobufCBufferSimple that are incorrect as of protobuf-c 1.0.0

protobuf-c 1.0.0 added a new 'allocator' field to ProtobufCBufferSimple
that controls memory allocation, which must be NULL in order to request
the default system allocator. Various instances of ProtobufCBufferSimple
in nmsg were allocated on the stack without zeroing the entire object,
which resulted in protobuf-c's memory allocation function dereferencing
a garbage pointer.

Always initialize ProtobufCBufferSimple objects.
input_seqsrc: Increase required verbosity level for noisy printf's

The "freeing old source", "initialized new seqsrc", and "resetting
source" debugging printf's generated by the seqsrc tracking code are
low-level, and noisy. They should require a higher verbosity level than
the debug message printed when a dropped container is detected.

Increase the required verbosity level for these messages from 5 to 6.
configure.ac: bump version to 0.9.1
ChangeLog: 0.9.1
Merge tag 'tags/v0.9.1' into debian
debian/control: update Maintainer field
debian/libnmsg6.symbols: update for 0.9.1
debian/changelog: 0.9.1-1