Older versions of Windows don't (always?) call VDDInitialize(), so we need
to statically initialize the uart struct.

This should address the problem reported by Fzf (FQBBS) via DOVE-Net:

... When using the included SBBSEXEC.DLL, the UART emulation
defaults to base address 0x000 and interrupt 0x0.  The lockups do not occur
when this is left as-is but none of the UART emulation does anything.  Setting
a ComPort in a [UART] section of SVDM.INI or SBBSEXEC.INI does change the I/O
address and IRQ to an expected value.  But this isn't necessary.  The mere
existence of one of those two INI files, even if empty, sets the UART to the
expected default of 0x3F8/IRQ4.  The debug entry is always as follows when left
without any INI file:

[180] SBBS: Virtualizing UART (0x0, IRQ 0)

This bug appears to be caused by commit 9266b997 (1 year ago), where
uart was no longer statically-initialzed and relied on VDDInitialize() to be
called to copy default_uart to the uart struct.