Add backendType to PGPROC, replacing isRegularBackend

Add backendType to PGPROC, replacing isRegularBackend

We can immediately make use of it in pg_signal_backend(), which
previously fetched the process type from the backend status array with
pgstat_get_backend_type_by_proc_number(). That was correct but felt a
little questionable to me: backend status should be for observability
purposes only, not for permission checks.

Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/b77e4962-a64a-43db-81a1-580444b3e8f5@iki.fi