userdb: Add support for receive-only users

Some use cases, like receive-only MTAs, need domain users for receiving
emails, but have no real need for passwords since they will never use
submission.

Today, that is not supported, and those use-cases require the
administrator to come up with a password unnecessarily, adding
complexity and possibly risk.

This patch implements "receive-only users", which don't have a valid
password, thus exist for the purposes of delivering mail, but always
fail authentication.

See https://github.com/albertito/chasquid/issues/44 for more details and
rationale.

Thanks to xavierg who suggested this feature on IRC.

docs/man/chasquid-util.1

@@ -133,7 +133,7 @@
 .\" ========================================================================
 .\"
 .IX Title "chasquid-util 1"
-.TH chasquid-util 1 "2023-07-29" "" ""
+.TH chasquid-util 1 "2023-12-03" "" ""
 .\" For nroff, turn off justification.  Always turn off hyphenation; it makes
 .\" way too many mistakes in technical documents.
 .if n .ad l
@@ -142,7 +142,7 @@
 chasquid\-util \- chasquid management tool
 .SH "SYNOPSIS"
 .IX Header "SYNOPSIS"
-\&\fBchasquid-util\fR [\fIoptions\fR] user-add \fIuser@domain\fR [\-\-password=\fIpassword\fR]
+\&\fBchasquid-util\fR [\fIoptions\fR] user-add \fIuser@domain\fR [\-\-password=\fIpassword\fR] [\-\-receive_only]
 .PP
 \&\fBchasquid-util\fR [\fIoptions\fR] user-remove \fIuser@domain\fR
 .PP
@@ -160,9 +160,12 @@ chasquid\-util \- chasquid management tool
 chasquid-util is a command-line utility for \fBchasquid\fR\|(1) operations.
 .SH "OPTIONS"
 .IX Header "OPTIONS"
-.IP "\fBuser-add\fR \fIuser@domain\fR [\-\-password=\fIpassword\fR]" 8
-.IX Item "user-add user@domain [--password=password]"
+.IP "\fBuser-add\fR \fIuser@domain\fR [\-\-password=\fIpassword\fR] [\-\-receive_only]" 8
+.IX Item "user-add user@domain [--password=password] [--receive_only]"
 Add a new user to the domain.
+.Sp
+If \fI\-\-receive_only\fR is given, then the user will never successfully
+authenticate. This is useful when creating receive-only users.
 .IP "\fBuser-remove\fR \fIuser@domain\fR" 8
 .IX Item "user-remove user@domain"
 Remove the user from the domain.

docs/man/chasquid-util.1.md

@@ -4,7 +4,7 @@ chasquid-util - chasquid management tool
 
 # SYNOPSIS
 
-**chasquid-util** \[_options_\] user-add _user@domain_ \[--password=_password_\]
+**chasquid-util** \[_options_\] user-add _user@domain_ \[--password=_password_\] \[--receive\_only\]
 
 **chasquid-util** \[_options_\] user-remove _user@domain_
 
@@ -24,10 +24,13 @@ chasquid-util is a command-line utility for [chasquid(1)](chasquid.1.md) operati
 
 # OPTIONS
 
-- **user-add** _user@domain_ \[--password=_password_\]
+- **user-add** _user@domain_ \[--password=_password_\] \[--receive\_only\]
 
     Add a new user to the domain.
 
+    If _--receive\_only_ is given, then the user will never successfully
+    authenticate. This is useful when creating receive-only users.
+
 - **user-remove** _user@domain_
 
     Remove the user from the domain.

docs/man/chasquid-util.1.pod

@@ -5,7 +5,7 @@ chasquid-util - chasquid management tool
 
 =head1 SYNOPSIS
 
-B<chasquid-util> [I<options>] user-add I<user@domain> [--password=I<password>]
+B<chasquid-util> [I<options>] user-add I<user@domain> [--password=I<password>] [--receive_only]
 
 B<chasquid-util> [I<options>] user-remove I<user@domain>
 
@@ -28,10 +28,13 @@ chasquid-util is a command-line utility for chasquid(1) operations.
 
 =over 8
 
-=item B<user-add> I<user@domain> [--password=I<password>]
+=item B<user-add> I<user@domain> [--password=I<password>] [--receive_only]
 
 Add a new user to the domain.
 
+If I<--receive_only> is given, then the user will never successfully
+authenticate. This is useful when creating receive-only users.
+
 =item B<user-remove> I<user@domain>
 
 Remove the user from the domain.