Add the LD_LIBRARY_PATH_FDS environmental variable.

This variable allows the loading of shared libraries via directory descriptors
rather than via library paths.  If LD_LIBRARY_PATH_FDS=3:4:12, the directories
represented by file descriptors 3, 4 and 12 will searched for shared libraries
before the normal path-based mechanisms are used.  This allows us to execute
unprivileged binaries from within a Capsicum sandbox even if they require
shared libraries.

Approved by:	rwatson (mentor)
Reviewed by:	kib
MFC after:	3 weeks
Sponsored by:	DARPA/AFRL
Test RTLD's new LD_LIBRARY_PATH_FDS variable.

Test LD_LIBRARY_PATH_FDS by linking a binary that requires a shared
library that isn't in any of the usual search paths.  Ensure this fails
when we don't supply LD_LIBRARY_PATH_FDS or we pass invalid information
in it.  Ensure it works when we pass the correct directory in various
places in the variable.

Approved by:	rwatson (mentor)
MFC after:	3 weeks
Sponsored by:	DARPA/AFRL