puffs_setfhsize() in init

fix more A0

branches: 1.16.6; 1.16.10;
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: 1.13.4;
* add mntfromname parameter to puffs_init()
* set it in file servers

use puffs_null_setops()

fix this also, had a similar rototill-typo as rot13fs

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().

support reclaim

use new style to indicate rootpath

* register fsync operation
* test for -s, which causes no detach (don't need to remember to
do follow-dances in gdb because of fork() used in daemon())

* initialize pflags properly
* make 'dump' imply 'don't detach'

example null file system built on top of the null layer in libpuffs