Python config cleanup

[ghost]

Motivation and Context

• Reducing dependencies between files, simplifying nested tests, and improving style consistency eases code maintenance.
• Reducing build-time dependencies helps with use cases like arc_summary: prefer python3 version and install when there is no python #8851 where Python is not available when building ZFS but the Python scripts are still desired.

Description

Don't require Python at configure/build unless building pyzfs.
Move ZFS_AC_PYTHON_MODULE to always-pyzfs.m4 where it is used.
Simplify nested tests to flat case pattern-matching statements.
Make test syntax more consistent.

How Has This Been Tested?

I've tested the following configurations:

• no options, with no python installed
• no options, with python3
• --without-python
• --disable-pyzfs
• --with-python=python2 --disable-pyzfs
• --with-python=3 --disable-pyzfs with no python installed
• --with-python=2 --disable-pyzfs with python3 installed

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.

[behlendorf]

@Conan-Kudo would you mind reviewing this.

[behlendorf]

@freqlabs one question, while I don't see anything bashspecific in this change. I wanted to make sure this was manually tested using an alternative like ash, dash, etc, since we've been bitten by that before.

[ghost]

@behlendorf I did my testing on FreeBSD where /bin/sh is a derivative of ash.