process-util: rework wait_for_terminate_and_warn() to take a flags parameter

This renames wait_for_terminate_and_warn() to
wait_for_terminate_and_check(), and adds a flags parameter, that
controls how much to log: there's one flag that means we log about
abnormal stuff, and another one that controls whether we log about
non-zero exit codes. Finally, there's a shortcut flag value for logging
in both cases, as that's what we usually use.

All callers are accordingly updated. At three occasions duplicate logging
is removed, i.e. where the old function was called but logged in the
caller, too.

src/systemctl/systemctl.c

@@ -3556,7 +3556,7 @@ static int load_kexec_kernel(void) {
                 _exit(EXIT_FAILURE);
         }
 
-        return wait_for_terminate_and_warn("kexec", pid, true);
+        return wait_for_terminate_and_check("kexec", pid, WAIT_LOG);
 }
 
 static int set_exit_code(uint8_t code) {
@@ -7057,9 +7057,9 @@ static int run_editor(char **paths) {
                 _exit(EXIT_FAILURE);
         }
 
-        r = wait_for_terminate_and_warn("editor", pid, true);
+        r = wait_for_terminate_and_check("editor", pid, WAIT_LOG);
         if (r < 0)
-                return log_error_errno(r, "Failed to wait for child: %m");
+                return r;
 
         return 0;
 }