unload.sh: support kexec unload when kexec_file_load is required

[tabraham]

Some systems, such as those using secure boot, require the use of the kexec_file_load syscall instead of kexec_load.

Using kexe -a -p -u will try the new kexec_file_load syscall first, and if it is not supported, fall back to using the old kexec_load.

[ptesarik]

Wow, yes, you're right. I never realized the unload was broken.