root: python variants use "python" binary

[ryandesign (Ryan Carsten Schmidt)]

The python26 and python27 variants in the root port rely on there being a "python" binary of that version. This is only the case on specific versions of Mac OS X (python26: Snow Leopard; python27: Lion), or if the user has used "sudo port select python" to select that version. But it is improper for ports to rely on the user having done so. The port must not rely on a "python" binary; it must use the specific versioned python binary provided by the port whose python variant is selected.

This is discussed also in #31247.

If the developers of root will not provide an option for this, we can patch it.

[ryandesign (Ryan Carsten Schmidt)]

Duplicate of #31119.