make fseek detect and produce an error for invalid whence arguments

make fseek detect and produce an error for invalid whence arguments

this is a POSIX requirement. we previously relied on the underlying fd
(or other backend) seek operation to produce the error, but since
linux lseek now supports other seek modes (SEEK_DATA and SEEK_HOLE)
which do not interact well with stdio buffering, this is insufficient.
instead, explicitly check whence before performing any operations.