Invoke the kld_* event handlers from linker_load_file() and
linker_unload_file() rather than kern_kldload() and kern_kldunload(). This
ensures that the handlers are invoked for files that are loaded/unloaded
automatically as dependencies. Previously, they were only invoked for files
loaded by a user.

As a side effect, the kld_load and kld_unload handlers are now invoked with
the kernel linker lock exclusively held.

Reported by:	avg
Reviewed by:	jhb
MFC after:	2 weeks
Add item for pkg(8).
Fix doc build.

Sponsored by:	The FreeBSD Foundation
Fix readdir for the root directory on a FAT32 file system. The root
directory is like any subdirectory and as such needs to use a real
cluster number. To this end, keep a DE structure for the root in
the DOS_FS structure and populate it accordingly.

While here:
o   allow consecutive path separators by skipping them all.
o   add missing $FreeBSD$ keyword to dosfs.h.
Bump the loader version to 3.2 after various dosfs fixes (in particular).