If binaries lack a pintable, execve() can now reject them at startup

If binaries lack a pintable, execve() can now reject them at startup
by returning EINVAL, rather than at runtime when the first system call
occurs (and then probably dumping core).  Let's find out if there are
any surprising exceptions we were unaware of (would need a custom linker,
or a non-crt0 runtime)
errno discussions with kettenis, sthen, millert. Vague concensus it is
worth trying now.