Let apkbuild-lint check options again, and pass the pmbootstrap specific
options with the new CUSTOM_VALID_OPTIONS variable. Add a test case and
adjust pmb.helpers.lint.check to return the output of apkbuild-lint, so
we can properly test it.
Related: https://gitlab.alpinelinux.org/Leo/atools/-/merge_requests/28
Fixes: pmaports#553
While at it, also remove unnecessary "#!/usr/bin/env python3" in files
that only get imported, and adjust other empty/comment lines in the
beginnings of the files for consistency.
This makes files easier to read, and makes the pmbootstrap codebase more
consistent with the build.postmarketos.org codebase.
