BUG: updated signal.buttord to be consistent with other filter design methods.

It now optimizes the order and natural frequencies such that the passband
is just met while maximizing the stopband (or vice versa for stopband filter).
TST: updated butterworth filter tests for update to signal.buttord function.

Also tightened tolerance to 1e-6 on butterworth test_bandstop.
Merge branch 'pr-3235' into master.

Reviewed at https://github.com/scipy/scipy/pull/3235