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.
c5bc872f5c