readline(): Add ability to modify a string buffer

If the 'buf' parameter is a non-0-length string, its contents will be
edited.  Previously, the initial contents of 'buf' were ignored and the
user entered its contents from scratch.

This change is necessary to support the upcoming "editenv" command but
could also be used for future commands which require a user to modify
an existing string.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>