Add File.Flush methodmap (#1056)

This allows us to do FileHandle.Flush() instead of FlushFile(FileHandle) and maintain consistency in always using methodmaps for file I/O operations.

core/logic/smn_filesystem.cpp

@@ -1195,6 +1195,7 @@ REGISTER_NATIVES(filesystem)
 	{"File.WriteLine",			sm_WriteFileLine},
 	{"File.EndOfFile",			sm_IsEndOfFile},
 	{"File.Seek",				sm_FileSeek},
+	{"File.Flush",				sm_FlushFile},
 	{"File.Position.get",		sm_FilePosition},
 	{"File.ReadInt8",			File_ReadTyped<int8_t>},
 	{"File.ReadUint8",			File_ReadTyped<uint8_t>},

plugins/include/files.inc

@@ -233,6 +233,13 @@ methodmap File < Handle
 	// @return                True on success, false otherwise.
 	public native bool Seek(int position, int where);
 
+	// Flushes a file's buffered output; any buffered output
+	// is immediately written to the file.
+	// 
+	// @return              True on success or use_valve_fs specified with OpenFile,
+	//                      otherwise false on failure.
+	public native bool Flush();
+	
 	// Get the current position in the file; returns -1 on failure.
 	property int Position {
 		public native get();