kexec-tools: Reset getopt before falling back to legacy syscall

The modules may need to parse the arguments again after
kexec_file_load(2) failed, but getopt is not reset.

This change fixes the --initrd option on s390x. Without this patch,
it will fail to load the initrd on kernels that do not implement
kexec_file_load(2).

Signed-off-by: Petr Tesarik <ptesarik@suse.com>
Signed-off-by: Simon Horman <horms@verge.net.au>