Fix: GCC warns about possibly uninitialized data in signal.cpp

When compiling with '-Og', GCC warns about variables that are
initialized by reference in the condition of a 'while' loop.

This commit silences the warning by explicitly initializing the
variables in question to their respective 'invalid value' markers,
which will most likely be optimized out when the compiler realizes
the values are never used.

(svn r16909) -Fix [FS#2996]: NewGRF stations would be triggering assertions all over the place when using the more advanced station types.
-Change: make (rail) waypoints sub classes of 'base stations', make buoys waypoints and unify code between them where possible.

(svn r14258) -Codechange: rework the way to query the vehicle hash to make sure it always results in the same irregardless of the order of the hash-linked-list.
-Fix: desync in PBS reservation following, vehicle flooding and road vehicle overtake/follow code.

(svn r11802) -Fix [FS#716]: do not crash trains when leaving depot to a very long track
-Codechange: use dedicated pathfinder for signal updating, resulting in better performance and possible future improvements