added FILENAME arg to crashwrite
we still need to generate pktnum even with ARG_FILENAME
reformat argument list
char -> uint8_t
output packet name

This allows a shell script to get the generated packet
name.
added tests for named packets
Merge branch 'feature/crashwrite' into develop

This branch adds the FILENAME argument to crashwrite to write a packet
into an explicit filename.  Crashwrite now emits the packet name on
stdout, where it can be read by shell scripts, etc.