Welcome the new user-land implementation of the pbi runtime
functionality. There is also a related patch to rtld.c which we will
be putting into our freebsd fork.

This patch replaces all the previous nullfs/fusefs VFS implementations.

Instead for PBI runtime, we will use PBI_PRELOAD to dynamically load a
wrapper library, and do the /usr/local -> /usr/pbi path mapping all in
userspace. This bypasses all the kernel VFS calls, which greatly speeds up
the execution time of PBIs, while improving reliability. (No more kernel
panics when VFS ops go haywire).

Required rtld.c patch:

https://github.com/pcbsd/freebsd/commit/956ccdbd6caa186658b95bcea18f41a50a901d73