Require EFT shared objects have a PT_PHDR entry to be considered

Require EFT shared objects have a PT_PHDR entry to be considered
a pie binary. The kernel will now reject executing a typical shared
library with EINVAL. This breaks compatibility with initial static pie
binaries and requires a recent user-land prior to upgrading. In
addition, more fine grained errors can be returned from execve(2)
when errors occur while attempting to execute ELF objects.

okay guenther@, kettenis@, deraadt@