sysinstall: send out READY=1 when we start our thing

This is useful so that whoever invokes the tool knows that the general
parameters checked out, and the tool now proceeds with actually doing
the work.

src/sysinstall/sysinstall.c

@@ -4,6 +4,7 @@
 #include <sys/stat.h>
 #include <unistd.h>
 
+#include "sd-daemon.h"
 #include "sd-json.h"
 #include "sd-varlink.h"
 
@@ -2239,6 +2240,10 @@ static int run(int argc, char *argv[]) {
 
         putchar('\n');
 
+        /* All questions asked, we are about to begin the installation. Let the service manager know (it's
+         * fine if we sent READY=1 before already, e.g. from the device auto-pick logic). */
+        (void) sd_notify(/* unset_environment= */ false, "READY=1");
+
         r = sysinstall_context_run(&context);
         if (r < 0)
                 return r;