Further TLS async work

Further TLS async work

* gnutls.c (boot_error): New function to either signal an
error or return an error code.
(Fgnutls_boot): Don't signal errors when running asynchronously.

* process.h (pset_status): Move here from process.c to be
able to use from gnutls.c.

* process.c (connect_network_socket): Do the TLS boot here
when running asynchronously.
(wait_reading_process_output): Rework the dns_processes
handling for more safety.