Ensure that 'sign' does not make any changes to assume database.
(1) in $SIGN, revert previous change (commit 3137b946) to call CLEARSIGN,
which has the unhappy side-effect of killing off stuff introduced
by asksign (which is supposed to persist)
(2) in SIGN-MEXPT, cut out calls to functions which push facts onto
*LOCAL-SIGNS* (namely TDZERO, TDPN, etc)
* tests/rtest_sign.mac, tests/rtest_ask.mac:
collect tests which require interaction into rtest_ask, and tests
which do not, into rtest_sign.
remove rtest_ask (interactive tests) from list of test scripts
(cherry picked from commit ff12e8bbaad95451ff1e3c1452bcfd5c380c7fb2)