MFC r288490: Add debug file extension to kldxref(8)

MFC r288490: Add debug file extension to kldxref(8)

  After r288176 [in head] kernel debug files have the extension .debug.
  They also moved [in head] to /usr/lib/debug/boot/kernel by default so
  in the normal case kldxref does not encounter them.  A src.conf(5)
  setting may be used to continue installing them in /boot/kernel
  though, so have kldxref skip .debug files in addition to .symbols
  files.

Merged this change to avoid warnings when a stable/10 kldxref runs
against a head install, perhaps on an upgrade to 11-CURRENT.  The change
to kernel debug files will not be merged to stable/10.

git-svn-id: svn://svn.freebsd.org/base/stable/10@298494 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f