Fix crashing bug with pcap_next{_ex}() and savefiles.

Now that we're initializing the ops vector of a pcap_t for a savefile in
savefile.c, we need to export pcap_oneshot() from pcap.c and set the
one-shot op to it for savefiles.

Fix a typo in a comment while we're at it.
snprintf() error to proper buffer

In fail label we're freeing pcap_t structure, so don't use p->errbuf,
but passed argument buffer.
Merge pull request #311 from darkjames/pcapng-error-fix

pcapng save format: snprintf() error to proper buffer
Document nanosecond-resolution pcap files.