Fixes KSP-KOS#2861 by replacing ProcessOneInputChar()'s use of
defaulted optional parameters with just having different
explicit signatures instead.  This way previously
compiled DLLs will pick the right one, which they cannot
do with defaulted args because in C# defaulted args are
appended at compile-time, not run-time or load-time. If the
signature required appending one extra optional arg back
when the compiler ran to build the caller's DLL, but it now
would need to append two such args, the DLL will still be
trying to get away with just appending one.  Thus why the
kOSPropMonitor DLL still insists on a 3-arg method existing
or it won't work.

tl;dr - adding more parameters but making them defaulted only
provides backward compatibility when all DLLs that call the
method are getting recompiled from scratch.  It doesn't when
the caller is a DLL that's not being recompiled.