Add optional `rtol` argument to 1D solvers

This exposes an option to control the relative tolerance of the 1D
root-finding functions in scipy.optimize. The feature existed in the C
implementations but was not exposed by the Python interface.
Enforce minimum value of rtol

This prevents a possible infinite loop.
Enforce minimum value of xtol

This prevents a possible infinite loop.
Merge pull request #2462 from lpsinger/scipy_optimize_rtol

Add optional `rtol` argument to 1D solvers