bootstrap: Add workaround for chflagsat() from 4.0.

bootstrap: Add workaround for chflagsat() from 4.0.

Avoid failing syscalls during installworld while updating from DragonFly 4.0.
Add special private version of chflagsat() to aid while performing migration.
Add triple protection for this private implementation variant.
Hopefully chflags-compat.h would never need any newer additions.

Since there are other issues that require both installkernel and installworld,
standard installkernel, reboot and installworld is not possible while updating
to recent DragonFly releases. chflags(1) must be handled with care.

Tested-on: kern.osreldate == 400000 updating straight to master (4.7).
  installworld step fail only on single non fatal chflags: invalid flag: xlink
  For make upgrade to work sendmail must be removed from /etc/mail/mailer.conf
  reboot(8) must be used cause shutdown is only functional only on new kernel.

Submitted-by: dillon@