Refactored snoopy enabling/disabling:
- moved snoopy-enable.sh into snoopy-enable
- created snoopy-disable
- both are now installed in sbin/
- snoopy path is now hardcoded into them at ./configure time
- added 'make disable' option
- 'make uninstall' now also executes 'make disable'
- configure.ac: require sed
- configure.ac: added some AC_SUBST macros that are required by features listed above