xmllint opt-out flag

Description

As discovered in #601, xmllint is always being checked and run if found. This creates an implicit dependency on xmllint that shouldn't be required for all runs of capgen.

Solution

Add an explicit flag to disable xmllint-ing and associated handling for checking if xmllint is available in the environment if enabled. This way if the flag is not turned on and xmllint is not found in the environment, an error can be generated instead of silently failing and just printing to stdout.

Alternatives (optional)

Discussed an opt-in flag but most uses cases are going to require linting.

Related to (optional)

#601