rlgwebd.js: prevent races when reading ttyrecs. DglSession objects read a 12-byte TTYREC header, extract therefrom the length of the data chunk, and then read the data. In between these two reads, the file watcher could trigger another readchunk() invocation, which might attempt to read a header from the beginning of the data chunk. This usually results in expecting a data chunk of several GB and failing to create a Buffer for it. The race is remedied by setting a flag on the DglSession object whenever readchunk() is called, clearing it when both reads complete, and refusing to read if it is already set.