Fix VM FOpenFile( FS_READ ) files seeking twice in FS_Seek
Remove unused variable baseOffset from fsh[]
Support FS_SEEK_END and negative offset for zipped files in FS_Seek

Use FS_SEEK_END in sound code instead of working around it.
If FS_SEEK_SET and going to current position, just return.
Remove old comment that FS_Seek doesn't work on zipped files
Remove function prototypes for non-existant Sys_* DLL functions