Letters reserved for future use

[Jim Blandy]

In (gdb)Remote Serial Protocol, every upper- and lower-case letter
that is not given a meaning is marked individually as "reserved for
future use".  This is kind of dopey.  Could we just have one remark at
the top that says that all upper- and lower-case letters not defined
there are reserved for future use?

What should people looking to make vendor-specific extensions to the
protocol do?  Are they required to stay within the general query /
general set commands, with a vendor prefix?