recovery: Support for compressed backups

 * Systems are getting quite large nowadays, and performing backups
   without using compression is wildly inefficient. A good compression
   algorithm also gives us a big speedup on backup and restore times since
   the process is entirely I/O bound.
 * Bring in pigz (via libminipigz), which is a multithreaded version of
   gzip and pipe tar backups and restores thru it.
 * Autodetect compressed images where appropriate, and add a menu option
   to select the compressed format by default.
 * Also fixed various trivial warnings and const-correctness issues.

Comparison on my Flo device right now:

Uncompressed:
  1868512   2013-08-24.07.10.08

Compressed:
  1149300   2013-08-24.06.51.14

Change-Id: I55025c993662142f5b647e600daf6885f9be6667