BUG/MINOR: thread: don't turn reversed thread ranges into "even"

BUG/MINOR: thread: don't turn reversed thread ranges into "even"

parse_thread_set() checks that the last thread number of a range is within
the permitted limits, but not that it is greater than or equal to the first
one. When storing the set, "max < min" is the internal encoding of "all"
(max=0), "odd" (max=-1) and "even" (max=-2), so any range written in
reverse order such as "thread 4-2" or "thread 1/4-2" on a "bind" line was
silently accepted and stored as "even" (memset 0xaa). With "nbthread 4",
"thread 4-2" binds the listener to threads 2 and 4 instead of threads 2 to
4, and "thread 4-1" also gives threads 2 and 4. There is no warning, so the
only visible effect is that some threads never accept connections on that
listener, which is easy to miss and hard to diagnose.

The other range parsers ("cpu-map", "process" via parse_process_number(),
and the cpuset parser) swap a reversed range, so let's do the same here.
This keeps configurations that rely on such ranges working, with the threads
the user wrote. A reg-test is added to check that "2-4", "4-2" and "1/4-2"
give the same thread mask.

This bug was introduced with the thread_set API in 2.8 (commit bef43dfa6
"MINOR: thread: add a simple thread_set API"), so this fix may be backported
to all stable branches down to 2.8.