doc open-file binary mode

[Kevin Ryde]

A few words for the "b" in open-file.  slib seems to make use of this,
so unless it's supposed to be a secret it may as well be described.

    `b'
          Use binary mode.  On DOS systems the default text mode
          converts CR+LF in the file to newline for the program,
          whereas binary mode reads and writes all bytes unchanged.  On
          Unix-like systems there is no such distinction, text files
          already contain just newlines and no conversion is ever made.
          The `b' flag is accepted on all systems, but has no effect
          on Unix-like systems.

          (For reference, Guile leaves text versus binary up to the C
          library, `b' here just adds `O_BINARY' to the underlying
          `open' call, when that flag is available.)