Avoid the layering violation of re-parsing for \E in roff_expand().

Avoid the layering violation of re-parsing for \E in roff_expand().
To that end, add another argument to roff_escape()
returning the index of the escape name.
This also makes the code in roff_escape() a bit more uniform
in so far as it no longer needs the "char esc_name" local variable
but now does everything with indices into buf[].
No functional change.