Add kern_shm_open().

This allows you to specify the capabilities that the new file descriptor
should have. This allows us to create shared memory objects that only
have the rights we're interested in.

The idea behind restricting the rights is that it makes it a lot easier
for CloudABI to get consistent behaviour across different operating
systems. We only need to make sure that a shared memory implementation
consistently implements the operations that are whitelisted.

Approved by: kib
Obtained from: https://github.com/NuxiNL/freebsd