core: Add support for renaming credentials with ImportCredential=

This allows for "per-instance" credentials for units. The use case
is best explained with an example. Currently all our getty units
have the following stanzas in their unit file:

"""
ImportCredential=agetty.*
ImportCredential=login.*
"""

This means that setting agetty.autologin=root as a system credential
will make every instance of our all our getty units autologin as the
root user. This prevents us from doing autologin on /dev/hvc0 while
still requiring manual login on all other ttys.

To solve the issue, we introduce support for renaming credentials with
ImportCredential=. This will allow us to add the following to e.g.
serial-getty@.service:

"""
ImportCredential=tty.serial.%I.agetty.*:agetty.
ImportCredential=tty.serial.%I.login.*:login.
"""

which for serial-getty@hvc0.service will make the service manager read
all credentials of the form "tty.serial.hvc0.agetty.xxx" and pass them
to the service in the form "agetty.xxx" (same goes for login). We can
apply the same to each of the getty units to allow setting agetty and
login credentials for individual ttys instead of globally.

src/shared/bus-unit-util.c

@@ -1310,6 +1310,49 @@ static int bus_append_execute_property(sd_bus_message *m, const char *field, con
                 return 1;
         }
 
+        if (streq(field, "ImportCredentialEx")) {
+                r = sd_bus_message_open_container(m, 'r', "sv");
+                if (r < 0)
+                        return bus_log_create_error(r);
+
+                r = sd_bus_message_append_basic(m, 's', field);
+                if (r < 0)
+                        return bus_log_create_error(r);
+
+                r = sd_bus_message_open_container(m, 'v', "a(ss)");
+                if (r < 0)
+                        return bus_log_create_error(r);
+
+                if (isempty(eq))
+                        r = sd_bus_message_append(m, "a(ss)", 0);
+                else {
+                         _cleanup_free_ char *word = NULL;
+                        const char *p = eq;
+
+                        r = extract_first_word(&p, &word, ":", EXTRACT_DONT_COALESCE_SEPARATORS);
+                        if (r == -ENOMEM)
+                                return log_oom();
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to parse %s= parameter: %s", field, eq);
+                        if (r == 0)
+                                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Missing argument to %s=.", field);
+
+                        r = sd_bus_message_append(m, "a(ss)", 1, word, p);
+                }
+                if (r < 0)
+                        return bus_log_create_error(r);
+
+                r = sd_bus_message_close_container(m);
+                if (r < 0)
+                        return bus_log_create_error(r);
+
+                r = sd_bus_message_close_container(m);
+                if (r < 0)
+                        return bus_log_create_error(r);
+
+                return 1;
+        }
+
         if (streq(field, "LogExtraFields")) {
                 r = sd_bus_message_open_container(m, 'r', "sv");
                 if (r < 0)