Install integration scripts to libexec directory (/usr/lib/maddy)

We don't want to clutter /usr/bin/ with internal helpers, do we?

On start-up, the libexec directory is added to the PATH environment
variale. This allows reusing exec.LookPath logic, notably, to check
whether the script is executable.

dist/install.sh

@@ -24,4 +24,4 @@ install -Dm 0644 -t "$DESTDIR/$FAIL2BANDIR/filter.d/" fail2ban/filter.d/*
 install -Dm 0644 -t "$DESTDIR/$PREFIX/lib/systemd/system/" systemd/maddy.service systemd/maddy@.service
 
 install -Dm 0644 -t "$DESTDIR/$CONFDIR/integration/" integration/rspamd.conf
-install -Dm 0755 -t "$DESTDIR/$PREFIX/bin/" scripts/maddy-rspamd-hook
+install -Dm 0755 -t "$DESTDIR/$PREFIX/lib/maddy/" scripts/rspamd-hook

dist/integration/rspamd.conf

@@ -8,7 +8,7 @@
 #
 
 check {
-    command maddy-rspamd-hook {source_ip} {source_host} {sender} {
+    command rspamd-hook {source_ip} {source_host} {sender} {
         code 1 reject
         code 2 quarantine
         code 3 reject 450 4.7.0 "Message rejected due to a local policy"

man/maddy-filters.5.scd

@@ -668,7 +668,8 @@ command executable_name arg0 arg1 ... {
 ## Arguments
 
 The module arguments specify the command to run. If the first argument is not
-an absolute path, it is looked up in $PATH. Note that no additional handling
+an absolute path, it is looked up in the Libexec Directory (/usr/lib/maddy on
+Linux) and in $PATH (in that ordering). Note that no additional handling
 of arguments is done, especially, the command is executed directly, not via the
 system shell.
 
@@ -776,3 +777,7 @@ for the message.
 
 This directives specified the mapping from the command exit code _integer_ to
 the message pipeline action.
+
+Two codes are defined implicitly, exit code 1 causes the message to be rejected
+with a permanent error, exit code 2 causes the message to be quarantined. Both
+action can be overriden using the 'code' directive.

run.go

@@ -55,6 +55,8 @@ func Run() int {
 
 	initDebug()
 
+	os.Setenv("PATH", config.LibexecDirectory+string(filepath.ListSeparator)+os.Getenv("PATH"))
+
 	f, err := os.Open(*configPath)
 	if err != nil {
 		systemdStatusErr(err)