uipc_shm: Add racct support

[cyril_freebsdfoundation.org]

Summary

This adds racct support for POSIX shared memory objects. They are now counted in the racct "nshm" and "shmsize" resources.

[cyril_freebsdfoundation.org]

Use new racct_add_cred_checked function instead of racct_add on resize. This is the correct behaviour, since a process might be able to resize a shared memory object that was created by a different process.