Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Explicitly pass in the relocation base and data offset into ef_reloc()
rather than relying on a trick that happens to work for the current
relocation schemes. Also add some comments and improve variable
naming.

Call the file format specific code through a table of function
pointers and remove knowledge of the file format from kldxref.c.
This will make it possible to support more than one file format.

Fix kldxref on sparc64 by allowing non-trivial relocations to be performed
on variables read out of raw kld files. Unlike other platforms the value
will be in an Elf_Rela, not in the data section of the elf file.

Submitted by: Hartmut Brandt <brandt@fokus.gmd.de>
PR: 46730
Tested on: alpha (obrien), i386, sparc64