Switch reorder_kernel() from sending emails to logging via syslog

Switch reorder_kernel() from sending emails to logging via syslog
and to write the logfile inside the kernel compile dir.

- turn the whole reorder_kernel function into a subshell {} -> ()
- create kernel compile dir early on
- redirect all stdout/stderr to a logfile inside this dir
- setup ERR trap handler that
  - disables the EXIT trap handler
  - syslogs the error and hints to the logfile
  - additionally sends this message to the console
- setup EXIT trap handler that syslogs success
- wipe only the content instead of the whole kernel compile dir
- reestablish stdout redirection to the log after the wipe
- remove -q option of sha256 to log check result
- run reorder_kernel() in the background

OK deraadt@ tb@