fix build of puffs examples; need _KERNTYPES for register_t

Gur Zbba vf Jnavat Perfprag (19% bs Shyy)

Add a little layering experiment, which both exercises the
componentname modification feature of libpuffs and tries to do
layering (just a little too hardcoded for comfort, fow now). This
simply uses the puffs nullfs to present all data and paths in rot13.
You can even mount it twice for extra fun.

fix more A0

branches: 1.16.6; 1.16.10;
Rototill.

Ok, ok, a few more words about it: stop holding puffs_cc as a holy
value and passing it around to almost every possible place (popquiz:
which kernel variable does this remind you of?). Instead, pass
the natural choice, puffs_usermount, and fetch puffs_cc via
puffs_cc_getcc() only in routines which actually need it. This
not only simplifies code, but (thanks to the introduction of
puffs_cc_getcc()) enables constructs which weren't previously sanely
possible, say layering as a curious example.

There's still a little to do on this front, but this was the major
fs interface blast.

use puffs_daemon() instead of daemon()

Actually, daemonize the file servers before mounting. I might require
at some point that the local protocol handler does not change after
the file system has been mounted.

Pull the daemonizing code out of the library mainloop into the file
servers. Calling daemon() (i.e. fork()ing) inside a library can
cause nice surprises for e.g. threaded programs. As discussed with
Greg Oster & others.

branches: 1.12.4;
* add mntfromname parameter to puffs_init()
* set it in file servers

Just to demonstrate namemod a bit further, make this flip the case
of filenames / contents instead of rot13 if -f is given.

use puffs_null_setops()

fix typo from previous rototill, makes this work again

Move puffs to a two clause license where it already isn't so. And
as agc pointed out, even files with the third clause were already
effectively two clause because of a slight bug in the language...

reflect changes in fs mount: use puffs_mount() instead of
puffs_domount() & puffs_start()

allow to mount nullfs types only from directories
TODO: generalize

Instead of keeping on adding parameters to puffs_mount(), make it
only take the bare essentials, which currently means removing
"maxreqlen" from the argument list (all current callers I'm aware
of set it as 0 anyway). Introduce puffs_init(), which provides a
context for setting various parameters and puffs_domount(), which
can be used to mount the file system. Keep puffs_mount() as a
shortcut for the above two for simple file systems.

Bump development ABI version to 13. After all, it's Friday the 13th.
Watch out! Bad things can happen on Friday the 13th. --No carrier--

Hide struct puffs_usermount from the rest of the world and provide
accessors for interesting data in it. Namely, you can now get
pu->pu_privdata with puffs_getspecific(), pu->pu_pn_root with
puffs_set/getroot() and pu->pu_maxreqlen with puffs_getmaxreqlen().

adapt to new readdir signature. no functional change

do reclaim

Gur Zbba vf Jnavat Perfprag (19% bs Shyy)

Add a little layering experiment, which both exercises the
componentname modification feature of libpuffs and tries to do
layering (just a little too hardcoded for comfort, fow now). This
simply uses the puffs nullfs to present all data and paths in rot13.
You can even mount it twice for extra fun.