* use full package list when checking for upgrades
 * verify sha256sums and usign signature of sha256sums.sig
 * introduce '-c' option to only check if system is up-to-date
 * introduce '-F' option to ignore the signature verification result
 * return -1 on locally caused and -2 on server-side errors
 * don't include locally appended attributes in debug output of a
   server reply
 * output bug report note on 412 target not found
 * use content-length header instead of filesize field in JSON content
 * suppress duplicate error messages
 * drop unused attributes
 * make debug messages optional at compile-time

Signed-off-by: Daniel Golle <daniel@makrotopia.org>