core: accept "|" ExecStart= prefix to spawn target user's shell

When switching to another user it's oftentimes desirable to also spawn
the target user's shell. sudo supports this via -i flag, run0 currently
doesn't. We don't want to proactively query NSS ourselves, since
that would fall short when operating remotely. Let's instead teach
the service manager to spawn the command using the user's default shell.

I opted for "|" instead of "." in the end because the latter seems
a bit obscure. But happy to change it to something else if a better option
comes up.

src/shared/exec-util.c

@@ -487,6 +487,7 @@ static const char* const exec_command_strings[] = {
         "privileged",     /* EXEC_COMMAND_FULLY_PRIVILEGED */
         "no-setuid",      /* EXEC_COMMAND_NO_SETUID */
         "no-env-expand",  /* EXEC_COMMAND_NO_ENV_EXPAND */
+        "via-shell",      /* EXEC_COMMAND_VIA_SHELL */
 };
 
 assert_cc((1 << ELEMENTSOF(exec_command_strings)) - 1 == _EXEC_COMMAND_FLAGS_ALL);