Improve reorder_kernel()

Improve reorder_kernel()

- check for and exit if /usr/share is on a nfs mounted filesystem
- add trap handlers that mail the logfile to the admin user
- use $_compile instead of $_compile_dir like in the installer
- use $_compile/$_kernel instead of $_kernel_dir
- remove the now redundant sha256 -h ... after make newinstall
- write stdout/stderr of the background subshell to a logfile

OK tb@ deraadt@