Fixed marking of access time for special files and fifos (don't do
it if the file system is mounted noatime). Not fixed: the access
time is marked at the start of a read() and not marked on successful
completion. I think this should be handled at the vfs level.

Print a better panic message for missing vops. Don't use printf()
before panic(), since the printf()ed part isn't shown by gdb.
This actually loses a little with the current gdb, since gdb just
prints the fmt arg to panic, so %'s aren't expanded. gdb should
fetch the full message from the message buffer if possible.

Fixed default vop function for vop_getpages_desc. It needs to
just return EOPNOTSUPP so that the vnode pager can get the pages
in using a general method. Panicing broke exec'ing of files on
ext2fs file systems. ffs works because it doesn't use the default.

Fixed nearby style bugs.