chasquid: Fail at RCPT TO time if a user does not exist

It's more convenient and in line with standard practice to fail RCPT TO if the
user does not exist.

This involves making the server and client aware of aliases, but it doesn't
end up being very convoluted, and simplifies other code.

test/t-01-simple_local/run.sh

@@ -7,6 +7,7 @@ init
 
 generate_certs_for testserver
 add_user testserver user secretpassword
+add_user testserver someone secretpassword
 
 mkdir -p .logs
 chasquid -v=2 --log_dir=.logs --config_dir=config &
@@ -25,6 +26,11 @@ if ! run_msmtp -a smtpport someone@testserver < content 2> /dev/null; then
 	exit 1
 fi
 
+if run_msmtp nobody@testserver < content 2> /dev/null; then
+	echo "ERROR: successfuly sent an email to a non-existent user"
+	exit 1
+fi
+
 if run_msmtp -a baduser someone@testserver < content 2> /dev/null; then
 	echo "ERROR: successfully sent an email with a bad password"
 	exit 1

test/t-02-exim/run.sh

@@ -39,6 +39,7 @@ EXIMDIR="$PWD/.exim4" envsubst < config/exim4.in > .exim4/config
 
 generate_certs_for srv-chasquid
 add_user srv-chasquid user secretpassword
+add_user srv-chasquid someone secretpassword
 
 # Launch chasquid at port 1025 (in config).
 # Use outgoing port 2025 which is where exim will be at.