* implement el_get(EditLine *, int op, void *result), which does the
inverse of el_set()
* add EL_EDITMODE operation to el_set and el_get; if non zero editing
is enabled (the default).
* add "edit on | off" editrc command, which modifies EL_EDITMODE.
users can now add '*:edit off' in ~/.editrc as an advisory to
disable editing.
NOTE: at this time EL_EDITMODE is just an indication of the
state of the 'edit' command. It's up to the application to check
this after el_source() or el_parse() to determine if editing is still
required.
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