Search for librrpreload.so in bin/../lib (where 'bin' is the directory
containing the rr executable according to argv[0]) and pass the full path
name to LD_PRELOAD if found there, so that LD_LIBRARY_PATH doesn't need
to be configured by users who built rr but didn't install it.

If librrpreload.so is not found in bin/../lib, assume it has been installed
somewhere under LD_LIBRARY_PATH, and set LD_PRELOAD to use the bare
library name.