sysdump: ignore files inside /sys/kernel/debug to avoid hanging

sysdump: ignore files inside /sys/kernel/debug to avoid hanging

There are several files inside /sys/kernel/debug which aren't
safe for cat-ing, like for example
/sys/kernel/debug/hid/0003:80EE:0021.0001/events So ignore all
files inside /sys/kernel/debug to avoid a hanging sysdump
process.

While at it avoid read on already closed file-handle and redirect
error message to stderr if a file can't be opened/read.

Closes grml/grml#55