Using the system libedit means that libedit will again make absolute
movements on certain kinds of terminals.  Older versions of libedit
without EL_PROMPT_ESC miscalculate the length of the prompt.
Combined, this results in the cursor ending up in the wrong place.

We knew this was a possibility when we moved to system libedit, and
we're OK with it.  We certainly lived without this fancy prompt
feature for a long time.

This change puts back in place detection of the terminal type when
EL_PROMPT_ESC isn't available.  On these older versions of libedit,
we'll only enable the fancy features when we know that libedit will
only make relative movements and not tickle the bug.

This partially reverts commit:
b9b77bd6