Use <elf.h> in !_KERNEL code path.

If you use sys/param.h, you don't need sys/types.h

Collapse db_dwarf.h into db_sym.h

ok matthew@

Sync no-argument function declaration and definition by adding (void).
ok mpi@ millert@

Kill trailing whitespaces. No object change.

Return correct file name entry from DWARF line table

We run the DWARF line table program to generate each row of the table
until we find a row after the one we wanted, and then take the
previous row's entries. The code correctly took the previous row's
line number entry, but incorrectly took the current row's file name
entry. Notably, this caused DDB to report the wrong file names for
inlined calls to functions defined in header files.

ok mlarkin

Add basic DWARF line table decoder

Includes a stand-alone addr2line clone for userspace testing.

Tested extensively on amd64 and expected to eventually support other
architectures too. Importing now so further development/testing can
happen in-tree. Followup commits will add to the kernel build and
integrate into ddb.

positive feedback; no objections