cqlsh: look for cqlshlib relative to realpath

patch by thepaul; reviewed by slebresne for CASSANDRA-3767

i.e., the realpath of the cqlsh executable, instead of the path as
called, which could be a symlink. If cqlsh is called via symlink, we are
more likely to find cqlshlib relative to the real location than relative
to the symlink.
2 files changed