Added search path relative to running binary on Windows.

In contrast to *IX it doesn't make much sense to add compile time defined
search paths to Windows binaries: There's no standard path like /usr/local/bin
(and there are no symbolic links to link from there to another location).

On the other hand it's (again in contrast to *IX) easy for Windows binaries
to determine their own paths. Therefore it's appropriate to make use of that
to add run time defined default search paths.

src/common/searchpath.h

@@ -79,6 +79,12 @@ void AddSubSearchPathFromEnv (SearchPath* P, const char* EnvVar, const char* Sub
  * the environment variable value.
  */
 
+void AddSubSearchPathFromWinBin (SearchPath* P, const char* SubDir);
+/* Windows only:
+ * Add a search path from the running binary, adding a subdirectory to
+ * the parent directory of the directory containing the binary.
+ */
+
 int PushSearchPath (SearchPath* P, const char* NewPath);
 /* Add a new search path to the head of an existing search path list, provided
  * that it's not already there. If the path is already at the first position,