"pkgdcl" component description, because if they are specified for the
top-level system, they're not guaranteed to be triggered before any of
the components are loaded.
And they are needed to be run before gray-streams is loaded.

It allows specifying that the sub-process must have a different
control TTY even if stdin/stdout/stderr are pipes.
Add slot PTY to class PROCESS to allow I/O in the latter case.