Add system-wide tunables: cache ld.so.cache

Add system-wide tunables: cache ld.so.cache

The purpose of this change is twofold:

1. The ld.so.cache is cached in memory and only re-read if/when
   it changes on disk.  This allows us to have much more intensive
   security checks in the future, without impacting performance as
   much.  It also allows for cases where the cache is corrupted -
   we continue using the last valid one.

2. We break out the load/check logic so that the cache can be
   loaded independently of the library lookup, such as for
   code that only needs to look at the extensions.

Reviewed-by: Arjun Shankar <arjun@redhat.com>