(_AT_DATA_CALC_Y): Include unistd.h if available.

(_AT_DATA_CALC_Y): Include unistd.h if available.
Define 'alarm' to do nothing if unistd.h is not available.
Add a new rule "exp: '-' error;" to test the above change to
data/yacc.c.  Use 'alarm' to abort any test taking longer than
10 seconds, as it's probably looping.
(AT_CHECK_CALC): Test recovery from error in new grammar rule.
Also, the new yacc.c generates two fewer diagnostics for an
existing test.