btrfs-progs: libify some parts of btrfs-progs

btrfs-progs: libify some parts of btrfs-progs

External software wanting to use the functionality provided by the btrfs
send ioctl has a hard time doing so without replicating tons of work. Of
particular interest are functions like btrfs_read_and_process_send_stream()
and subvol_uuid_search(). As that functionality requires a bit more than
just send-stream.c and send-utils.c we have to pull in some other parts of
the progs package.

This patch adds code to the Makefile and headers to create a library,
libbtrfs which the btrfs command now links to.

Signed-off-by: Mark Fasheh <mfasheh@suse.de>
Signed-off-by: David Sterba <dsterba@suse.cz>