Remove spurious ABI tags from kdump output.

The abidump routine output an ABI tag when -A was specified for records
that were not displayed due to type or pid filtering. To fix, split
the code to lookup the ABI from the code to display the ABI, move the
code to display the ABI into dumpheader(), and move dumpheader() later
in the main loop as a simplification. Previously dumpheader() was
called under a condition that repeated conditions made later in the
main loop.

Reviewed by: kib
MFC after: 1 month
Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D16608