mount_9p: enable to communicate with vio9p via its character device file

With this feature, we can mount an exported filesystem by a VM host via
virtio-9p.

add missing header

branches: 1.7.28; 1.7.34;
adapt to new cmpfb signature

branches: 1.6.4;
Change the signature of puffs_respcmp to return -1,0,1 for smaller,
equal, larger, respectively instead of 0/1 for non/equal. This
will allow sorting the buffers for faster matching in libpuffs.
While here, change the name from respcmp to framecmp, as that better
reflects the purpose.

NOTE! there is no obvious way to make compilation fail for file
systems which may already be using this feature (although I don't
think there are any outside our tree, as the feature is two weeks
old). Nevertheless, non-updated file systems will fail very quickly.

don't leak memory if getting data from the buffer fails

CID 4485

adapt to "step 3" of puffs_framebuf changes

use puffs_framebuf_remaining()

Adapt to use puffs_framebuf. This gets rid of most of the duplicated
code between mount_psshfs and mount_9p and clarifies the code
structure.

mount_9p: mount a file server using the Plan9 file sharing protocol

Works, but lots of little things to nibble on:
* fix permissions to work better
* limit the amount of open files required
* do constant folding with psshfs code
* support authentication
etcetc.