Extend specint(exp(-s*t)*t^z,t) to complex z

Extend specint(exp(-s*t)*t^z,t) to complex z

For complex z, the real part of z needs to be greater than -1.  The
only change necessary to make this work is to update F1P137TEST so
it checks the sign of the real part of z+1 instead of the sign of
the entire expression z+1.

See also commit f49b1d1f which similarly extended laplace.

No problems with the test suite or share test suite.  Some new tests
have been added to rtest_hypgeo.