Read Microsoft FTP Service listings

[christhomas]

Fixes #43, #54 and #65 — the same cause, reported three times over four years.

IIS does not send an ls-style listing:

10-09-20  09:36PM       <DIR>          aspnet_client
10-16-20  05:20PM                 6989 Biography.html
03-02-2023  03:15PM       <DIR>          Archived Tracking

No permission bits, no owner, no link count. lsRegex matches none of it, so ReadDir fails outright on every IIS server — @dorimon-1 reported it as a panic in #65.

Three details that are easy to get wrong

• The year is two digits or four, in listings from the same server family (10-09-20 in Microsoft FTP Service #54, 03-02-2023 in ReadDir() cannot read windows hosted ftp directories #65). The layout follows the year's width rather than a pivot of our own: time.Parse already decides what a two-digit year means, and disagreeing with it here would be a second rule to maintain.
• A directory is marked <DIR> where a size would be. It gets no size, rather than zero dressed up as one — the server did not send a number.
• The name runs to the end of the line, so it may contain spaces. Archived Tracking is one entry, not two.

Every test entry is copied from one of the three reports, plus midnight and noon, which is where 12-hour clocks go wrong.

What it does not change

lsRegex is tried first and is untouched, so this adds a format rather than replacing one. parseDOSLIST returns (nil, nil) when the line is not in its format, so the caller reports the failure it was already going to report rather than replacing it with a less accurate one.

A line that is neither format is still rejected — including a DOS-shaped line with an impossible time, because the looser pattern must not swallow what the stricter one refused. That case is tested.

@dorimon-1 offered a PR for this in #65; happy to defer if that lands first.

One note on running the tests: this branch is off master, so TestMain still requires ./build_test_server.sh — these tests need no server, but they cannot start without one. I verified them via the container harness in #73.