Added {set,query}_{read,write,close}_callback() where missing.

Strengthened the type of set_nonblocking() somewhat.
Splitted the implementation of Stdio.TELNET.Readline into two parts.
Stdio.TELNET.Readline now utilizes subtyped objects to avoid callback confusion.
Added some Autodoc mk II markup.

Rev: lib/modules/Protocols.pmod/TELNET.pmod:1.33