Compact m_hdr by packing the type and flags fields into one uint32_t.

The mbuf type is an enumerator with only a handful of types in use and
thus reduced from int to 8bits allowing for 255 types to be specified.
Only 5 types have been in use for a long time.

The flags field gets the remaining 24 bits with 12 bits for global
persistent flags and 12 bits for protocol/layer specific overlays.
Some of the global flags/functionality can be moved to the csum_flags
or ext_flags bits in the future.

MT_VENDOR[1-4] and MT_EXP[1-4] types for vendor-internal and
experimental local mapping are added.

The size of m_hdr shrinks from 24/40 to 20/32bytes (32/64bit architectures).

Sponsored by:	The FreeBSD Foundation
Remove some code that has been commented out since it was added in 2000.
Do not use "Enable Implied Seek" on enhanced floppy controllers.  This
breaks the "2step" feature of the driver, e.g. in order to read 360
KiB media on a 1200 KiB drive.

As the only potential advantage of implied (vs. explicit) seeks is to
minimize the software effort, yet our driver always contained the
logic needed for explicit seeks, simply dropping implied seeks is the
best solution without introducing risks for new bugs.  There is no
performance penalty, reading a 1440 KiB medium takes exactly the same
time with both, implied or explicit seeks.

MFC after:	1 week
Remove the kld lock macros and just use the sx(9) API. Add locking in
linker_init_kernel_modules() and linker_preload() in order to remove most
of the checks for !cold before asserting that the kld lock is held. These
routines are invoked by SYSINIT(9), so there's no harm in them taking the
kld lock.
Set things up so that linker_file_lookup_set() is always called with the
linker lock held. This makes it possible to call it from a kld event handler
with the shared lock held.

Reviewed by:	jhb
Remove unused m_free_fast().  The difference to m_free() is only
2 predictable branches nowadays.  However as a pre-condition the
caller had to ensure that the mbuf pkthdr did not have any mtags
attached to it, costing some potential branches again.

Sponsored by:	The FreeBSD Foundation
Rename the kld_unload event handler to kld_unload_try, and add a new
kld_unload event handler which gets invoked after a linker file has been
successfully unloaded. The kld_unload and kld_load event handlers are now
invoked with the shared linker lock held, while kld_unload_try is invoked
with the lock exclusively held.

Convert hwpmc(4) to use these event handlers instead of having
kern_kldload() and kern_kldunload() invoke hwpmc(4) hooks whenever files are
loaded or unloaded. This has no functional effect, but simplifes the linker
code somewhat.

Reviewed by:	jhb