When using the escape sequence &~ the code path is client_loop() ->

When using the escape sequence &~ the code path is client_loop() ->
client_simple_escape_filter() -> process_escapes() -> fork() and the pledge for
this path lacks the proc promise and therefore aborts the process.
The solution is to just add proc the promise to this specific pledge.

Reported by Gregoire Jadi gjadi ! omecha.info
Insight with tb@, OK jca@