pnmdec: add support for mono images with non-space-separated pixel digits

When the file to decode contains a sequence of binary values like
"1101110...", decode_frame() was reading the sequence of digits like a
unique integer value, which was resulting in integer overflows.

The change add support for parsing non-space-separated pixel digits
for mono formats, in particular fix decoding of file battrace.pbm, and
fix trac issue #154.
