Currently we disable kbind(2) for static program from libc.a's

Currently we disable kbind(2) for static program from libc.a's
preinit hook.  Delete that and instead have the kernel disable kbind
at exec-time if the program doesn't have an ELF interpreter.  For
now, permit userland calls to disable it when already disabled so
existing static programs continue to work.

prompted by deraadt@ questioning about the call in libc.a

ok deraadt@ miod@