Allow FOSSIL mode of the Win32 virtual UART/FOSSIL driver to be disabled

When configuring an external program, a Win32 sysop can now choose to *only* enable the virtual UART (and not the FOSSIL driver) feature of the Synchronet virtual UART/FOSSIL driver (dosxtrn.exe/sbbsexec.dll). Some programs (e.g. TradeWars 2) will always use the FOSSIL driver if one is detected and in order to force the use of COM/UART driver for I/O (if that is desired), then one must disable the FOSSIL driver. It's a rare use case, but I can see the potential need.

Renamed the "Intercept I/O" option in SCFG to "I/O Method" and make it clear that "Socket" (for native programs) and "FOSSIL or UART" (for 16-bit DOS programs) is the default method (what was previously identified as "No" I/O Interception).

If a sysop want to disable the virtual UART support (on Win32, e.g. so *only* FOSSIL is available to one or all DOS programs), they do that via their sbbsexec.ini file.