Bugfix: Bad shift value in extmon pipe comms code.

Shifted a value by 8 too many bits.  Instances with less than
256 specific extmon commands would not have been affected, but
those with >= 256 would have experienced early startup failure
when initializing extmon communications.
Fix unaligned access issues

Note this relies on gcc __attribute__((__packed__)), which some
  non-gcc compilers also support.  Probably ok, could dive deeper
  on supporting other compilers if anyone ever files a bug.
unaligned access fixup
minor comment/style cleanup on prev two commits
Switch version policy to SemVer
bugfix for "none" output of compile-time feature list
README updates, mostly to mention the extmon plugin
Version bumps and NEWS update for the 1.8.1 release
Merge tag 'v1.8.1' into debian

Release 1.8.1
Set Architecture to "any" again as unaligned access issues were fixed.