XL Microseconds Fix 1.0

Makes File Sharing work on a Macintosh XL (Apple Lisa running MacWorks Plus II) under System 7.5.x.

Without it, turning File Sharing on bombs: "File Sharing Extension illegal instruction". The cause,
found with MacsBug: System 7.5's _Microseconds timer routine goes wrong under MacWorks, and File
Sharing calls it to time the CPU while it starts up. System 7.1's File Sharing never calls it, which
is why 7.1 always worked.

This 540-byte extension replaces _Microseconds with a simple version that counts from the
60-a-second tick clock. It installs only on a 68000 Mac running System 7.5 or later (the XL, or a
Plus or SE); on anything else it does nothing. It changes memory only, never the System file.

Files: XL_Microseconds_Fix_1.0.bin (MacBinary II of the extension), the Read Me as it ships, and SHA256SUMS.txt.