Rewrite. Now, this module is reentrant on systems

Rewrite.  Now, this module is reentrant on systems
that provide openat (Solaris), and on systems like Linux+procfs
where our openat emulation code is reentrant.  This also fixes a
few low-probability leaks and eliminates some code that could,
in very unusual circumstances, cause rm() (via a callee) to exit.