Generalize masquerade_target_user_access() to allow checks for any user, not just current user

Problem/Motivation

I want to create a form where I can list the users that can masquerade as a specific target user. However, masquerade_target_user_access() only has a parameter for the target user, and it assumes that I want to check the current user's access. This means I would need to copy the code into a custom function to allow checking for a different user.

Steps to reproduce

masquerade_target_user_access() doesn't have a parameter for user to check access for.

Proposed resolution

It would be simple to add a parameter and have it default to the current user if no value is passed.

Remaining tasks

User interface changes

API changes

Data model changes