usr merge fixups

Debian and Ubuntu have releases coming out with usr-merge in place. For these systems, /bin and /sbin are symlinks to their respective /usr directories. This breaks a few tests in the python utils and in the regression tests. This patch series fixes them, mostly by performing realpath() calls when necessary. For the ptrace regression test, it copies the called /bin/true binary into the created temporary directory and executes it from there. (Good for other reasons, too.)

These patches should be safe to backport to the supported releases (but I have not attempted that, yet).