The new PRELOAD_FDS variable accepts a list of file descriptors
that should be loaded into the process.
This may be used to optimize a loading process - in the case when
we already have a file descriptor to the library; we don't have
to look into multiple PATH to find it.
It may also be used in capability mode to load a single additional
library without the need to open a directory that contains it.
The last use of this functionality t may be a race-free method
of loading libraries.
Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D29334
f90218886f