Autoconf updates & sync with long-standing downstream config forks

- Re-organized to put system configuration first
- Removed included PCRE, now will use system's PCRE if specified
- Fixed the duplicate PCAP header check, broken by the libpcap 1.0 release
- Prefixed all tack-on autoconf variables from _FOO -> EXTRA_FOO
- Capitulated to Debian: use tar.gz instead of tar.bz2 :-(
- Corrected aesthetic typos in emitted configure messages WRT bpf.h
- Trailing whitespace murder
Add tcpkill/dsniff (-K), a downstream patch from Debian
Nuke included PCRE 7.4 library, deferring to system version
Documentation updates