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.