As with LeiToMail, we'll exclusively rely on O_EXCL and EEXIST
instead of "-f" (stat(2)) for file name collision checking.
Furthermore, we can rely on link(2) error handling instead of
using stat(2) to check the result of link(2).
We'll still keep the hostname in these filenames, but memoize it
on a per-instance basis since hostname changes are rare and we
can assume it won't change between "tmp" and "cur".
We'll also start embedding the PID as {"tmp.$$"} into the fiel
name to guard against accidental deletion in child processes,
instead of requiring an extra hash lookup.
Finally, avoid multiple getpid(2) syscalls in internal subs
since glibc no longer caches in getpid(3).
We'll also favor constant comparison of $! against EEXIST for
inlining. and stop doing ->autoflush when we only have a single
print + flush.