mountroot should probe file systems

[Simon 'corecode' Schubert]

When mounting root from hammer, it is necessary to specify the file 
system type in the vfs.root.mountfrom setting, otherwise the machine 
will just be unhappy and issue the mountroot prompt (or rather, directly 
go to ddb, see other bug report).  This is very inconvenient and 
irritating.  The kernel should try all available file systems to mount root.

Possibly this should also be extended to mount itself, but this bug 
report only deals with the mountroot issue.