fs-mistrust: strip whitespace on environment variable values

Mistrust reads an environment variable as part of determining if its checks should be enabled or not. There are some specific keywords that are considered falsy: "false", "no", "never", "n", "0", and the empty string.

A small footgun is that the strings aren't stripped, so FS_MISTRUST_DISABLE_PERMISSIONS_CHECKS="false " is evaluated as truthy whereas FS_MISTRUST_DISABLE_PERMISSIONS_CHECKS="false" is falsy.

I think it makes sense to strip leading and trailing ascii whitespace so that "false " is evaluated as falsy. I plan to do this after !2674 (merged).