fix: consolidate FILETIME conversions into wintime.py and fix qwinsta crash on Windows

[n3rada]

Hello maintainers 👋

Picking up on #2022 (and also fixing #1374 along the way).

The getUnixTime function was copy-pasted across 11 files with no single authoritative definition. Two of those copies used /= instead of //=, silently producing floats. The one in smbserver.py was a dead wrapper around smb.FTtoPOSIX that was never called. The same problem exists on the inverse side: POSIXtoFT and FTtoPOSIX lived in smb.py and were re-imported everywhere.

What this PR does:

Introduces impacket/wintime.py as the single home for Windows FILETIME conversions. It exposes three functions:

• filetime_to_datetime(t): converts a FILETIME value to a naive UTC datetime object by computing datetime(1601, 1, 1) + timedelta(microseconds=t // 10). Avoids fromtimestamp() entirely, so it never crashes on Windows for any input value. For t=0 (uninitialised/never-set fields) it returns datetime(1601, 1, 1) rather than lying with the Unix epoch.
• filetime_to_posix(t): converts to a POSIX integer for callers that genuinely need one (e.g. os.utime). Includes a zero-guard for values below the FILETIME epoch.
• posix_to_filetime(t): replaces POSIXtoFT.

All callers that previously did datetime.fromtimestamp(getUnixTime(t)) or datetime.fromtimestamp(filetime_to_posix(t)) now use filetime_to_datetime(t) directly. No legacy aliases remain anywhere. smb.py still imports from wintime for its own internal use but no longer re-exports the functions.

I chose a dedicated wintime.py rather than dropping this into smb.py or a generic utils.py — the SMB module is already large and these conversions are used by ese, nspi, tsts, dpapi, and several examples that have nothing to do with SMB.

Closes #2022
Closes #1374

Best regards