kernel: Fix computation of number of threads at checkpoint restore

Each note is directly preceeded by its header which is struct Elf_Note and
note's owner name (in case of checkpoint files either 'CORE' or 'DragonFly').

Checkpoint files contain: one note CORE:NT_PRPSINFO and three notes
(CORE:NT_PRSTATUS, CORE:NT_FPREGSET and DragonFly:NT_DRAGONFLY_TLS) per thread.
kernel/kern_checkpoint.c: Fix overwriting TLS data of each thread
kernel: Remove some unnecessary debug messages
kernel: ELF notes are always 4-byte aligned