BUILD: applet: mark the CLI's generic variables as deprecated

BUILD: applet: mark the CLI's generic variables as deprecated

The generic context variables p0/p1/p2, i0/i1, o0/o1 have been abused
and causing trouble for too long, it's time to remove them now that
they are not used anymore.

However the risk that external code still uses them is not nul and we
had not warned before about their removal. Let's mark them deprecated
in 2.6 and removed in 2.7. This will let external code continue to work
(as well as it could if it misuses them), with a strong encouragement
on updating it.

If you found this commit after a bisect session you initiated to figure
why you got some build warnings and don't know what to do, have a look
at the code that deals with the "show fd", "show env" or "show servers"
commands, as it's supposed to be self-explanatory about the tiny changes
to apply to your code to port it. If you find APPLET_MAX_SVCCTX to be
too small for your use case, either kindly ask for a tiny extension
(and try to get your code merged), or just use a pool.