If the initial attempt to open /dev/ksyms fails, kldload the ksyms module
and retry.
When reentering kdb, typically due to a bug causing trap or assert in
the code executed in the context of debugger, do not be ashamed to
inform loudly about the re-entry.  Also, print the backtrace before
obliterating current stack with longjmp, allowing the operator to see
a place which caused the bug.

The change should make it less mysterious debugging the ddb itself.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Move new pf includes to the pf directory. The pfvar.h remain
in net, to avoid compatibility breakage for no sake.

The future plan is to split most of non-kernel parts of
pfvar.h into pf.h, and then make pfvar.h a kernel only
include breaking compatibility.

Discussed with:		bz
Several small fixes for the amd64 minidump code.

In report_progress(), use nitems(progress_track) instead of manually
hard-coding array size.  Wrap long line.

In blk_write(), code verifies that ptr and pa cannot be non-zero
simultaneously.  The later check for the page-alignment of the ptr
argument never triggers due to pa != 0 always implying ptr == NULL.  I
believe that the intent was to ensure that physicall address passed is
page-aligned, since the address is (temporary) mapped for the duration
of the page write.

Clear the progress_track.visited fields when starting minidump.  If
minidump is restarted or taken second time during the system lifetime,
progress is not printed otherwise, making operator suspectible to the
dump status.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week