kernel: Use 'DragonFly' as owner of TLS notes

NT_DRAGONFLY_TLS is a type of note used only by DragonFly BSD. Therefore,
each node of that type should have custom owner "DragonFly" in order to
prevent any future conflicts.
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
kernel: call set_user_TLS() after restoring TLS configuration

Architecture dependent part of restoring TLS configuration is called by
cpu_heavy_restore but not by syscall exit routines. This means that
for the main thread (i.e. the one that takes care of restoring from checkpoint)
set_user_TLS() has to be explicitly called.