New APIs: compress-out, compress-device-out.

New APIs: compress-out, compress-device-out.

These APIs let you copy compressed files or devices out from the disk
image.

Compression is useful for large images which are mostly zeroes.  We
cannot currently do sparseness detection, and compression gives us a
form of zero detection for free.

Example usage:

$ guestfish --ro -a /dev/vg_pin/F16x64 -i \
    compress-out gzip /etc/passwd /tmp/passwd.gz
$ file -z /tmp/passwd.gz
/tmp/passwd.gz: ASCII text (gzip compressed data, was "passwd", from
Unix, last modified: Sun Aug 28 14:40:46 2011)