Only initialize libedit and history when in interactive mode.

[jmmv]

Summary

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.

Test Plan

Run the command above to see that the "staircase effect"
described in the PR is not a problem any longer. Also run the basics_test
test program to see that the terminal output is sane.

[eadler]

perhaps it would be better to hide libedit init under isatty instead of specific command line flags?

[jmmv]

I don't think so. That code path uses el_gets() unconditionally so we need libedit to be initialized. Not using libedit if not a tty would be out of scope for this change, and a larger change.