Fix a fallout with my xensuspend merge: talk_to_backend() returns a
boolean, so checking for "true" with "== 0" is... wrong.

Now xennet(4) should work as expected, and not stay in the InitWait state
(which blocks network communication with the backend).

Thanks to riz@ and sborrill@ for reporting breakage with -current
xennet(4) after my merge.
compute the entry size correctly.
move the size marker back to where it was, but fix the name.
Document new *ASSERTMSG() prototype, and update KASSERT(9).

See also
http://mail-index.netbsd.org/tech-kern/2011/09/07/msg011427.html
return the namemax from the bsd statvfs which is filesystem dependent, not
a random value.
use CODA_MAXNAMLEN instead of bogus MAXNAMLEN
use the filesystem specific namemax not a hard-coded MAXNAMLEN
define and use ISO_MAXNAMLEN instead of MAXNAMLEN
Modify *ASSERTMSG() so they are now used as variadic macros. The main goal
is to provide routines that do as KASSERT(9) says: append a message
to the panic format string when the assertion triggers, with optional
arguments.

Fix call sites to reflect the new definition.

Discussed on tech-kern@. See
http://mail-index.netbsd.org/tech-kern/2011/09/07/msg011427.html
use NFS_MAXPATHLEN instead of MAXPATHLEN
use NFS_MAXNAMLEN for all names.
use NAME_MAX instead of MAXNAMLEN
define and use TMPFS_MAXNAMLEN instead of MAXNAMLEN
- rename UDF_MAX_NAMELEN -> UDF_MAXNAMLEN
- use NAME_MAX instead of MAXNAMLEN
use NAME_MAX instead of MAXNAMLEN
remove unused constant
define FDESC_MAXNAMLEN and use it.
define KERNFS_MAXNAMLEN and use it.`
define PROCFS_MAXNAMLEN and use it.
define RUMPFS_MAXNAMLEN and use it.