Ask for more warnings during a Travis CI build 

.travis.yml: add 'touch .devel configure'
Travis script: refactor env section
don't include pcap.h needlessly

Both interface.h and netdissect.h include <pcap.h>, thus most files
should not include it regardless if these need it or not. The only
exceptions so far remain:
* addrtoname.c
* missing/datalinks.c
* missing/dlnames.c
* tcpdump.c
NDOize CIP, IPComp and IPFC decoders

Also remove some unneeded includes while at it.