Only initialize libedit when necessary

The code path to support units conversions from the command line
need not initialize neither libedit nor the history. Therefore, only do
that when in interactive mode.

This hides the issue reported in PR bin/201167 whereby running commands
of the form 'echo "$(units ft in)"' would corrupt the terminal. The real
issue causing the corruption most likely still remains somewhere.

PR: bin/201167
Differential Revision: D2935
Reviewed by: eadler