core/service: imply Type=exec if credentials are used

When credentials are used with Type=simple, i.e. when
multiple sd-executor instances are running in parallel
for a single service, the state of final credential dir
might be unexpected wrt path_is_mount_point() and other
steps. So, let's imply Type=exec if not explicitly specified.

man/systemd.service.xml

@@ -162,11 +162,11 @@
 
  <itemizedlist>
  <listitem><para>If set to <option>simple</option> (the default if <varname>ExecStart=</varname>
- is specified but neither <varname>Type=</varname> nor <varname>BusName=</varname> are), the
- service manager will consider the unit started immediately after the main service process has
- been forked off (i.e. immediately after <function>fork()</function>, and before various process
- attributes have been configured and in particular before the new process has called
- <function>execve()</function> to invoke the actual service binary). Typically,
+ is specified but neither <varname>Type=</varname> nor <varname>BusName=</varname> are, and
+ credentials are not used), the service manager will consider the unit started immediately after
+ the main service process has been forked off (i.e. immediately after <function>fork()</function>,
+ and before various process attributes have been configured and in particular before the new process
+ has called <function>execve()</function> to invoke the actual service binary). Typically,
  <varname>Type=</varname><option>exec</option> is the better choice, see below.</para>
 
  <para>It is expected that the process configured with <varname>ExecStart=</varname> is the main
@@ -188,7 +188,9 @@
  Note that this means <command>systemctl start</command> command lines for <option>exec</option>
  services will report failure when the service's binary cannot be invoked successfully (for
  example because the selected <varname>User=</varname> doesn't exist, or the service binary is
- missing).</para></listitem>
+ missing). This type is implied if credentials are used (refer to <varname>LoadCredential=</varname>
+ in <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+ for details).</para></listitem>
 
  <listitem><para>If set to <option>forking</option>, the manager will consider the unit started
  immediately after the binary that forked off by the manager exits. <emphasis>The use of this type

src/core/service.c

@@ -20,6 +20,7 @@
 #include "devnum-util.h"
 #include "env-util.h"
 #include "escape.h"
+#include "exec-credential.h"
 #include "exit-status.h"
 #include "fd-util.h"
 #include "fileio.h"
@@ -815,6 +816,8 @@ static int service_add_extras(Service *s) {
  /* Figure out a type automatically */
  if (s->bus_name)
  s->type = SERVICE_DBUS;
+ else if (exec_context_has_credentials(&s->exec_context))
+ s->type = SERVICE_EXEC;
  else if (s->exec_command[SERVICE_EXEC_START])
  s->type = SERVICE_SIMPLE;
  else