tmpfiles: add conditionalized execute bit (X) support

tmpfiles: add conditionalized execute bit (X) support

According to setfacl(1), "the character X stands for
the execute permission if the file is a directory
or already has execute permission for some user."

After this commit, parse_acl() would return 3 acl
objects. The newly-added acl_exec object contains
entries that are subject to conditionalized execute
bit mangling. In tmpfiles, we would iterate the acl_exec
object, check the permission of the target files,
and remove the execute bit if necessary.

Here's an example entry:
A /tmp/test - - - - u:test:rwX

Closes #25114