Seeks to a specific file offset within an open file.
Sometimes the library needs to know the length of a file. It does this by seeking to the end of the file with seek(file, 0, MSPACK_SYS_SEEK_END), then calling tell(). Implementations may want to make a special case for this.
Due to the potentially varying 32/64 bit datatype off_t on some architectures, the MSPACK_SYS_SELFTEST macro MUST be used before using the library. If not, the error caused by the library passing an inappropriate stackframe to seek() is subtle and hard to trace.