core/socket: introduce DeferTrigger= and DeferTriggerMaxSec=

Alternative to b50f6dbe57

The commit naively returned early from socket_enter_running(), which however
is quite problematic, as the socket will be woken up over and over again
without doing a thing, until we eventually hit Poll/TriggerLimit*=.
On top of that it requires hacks to hold the start job for initrd-switch-root.service
up. Overall I doubt that is the right approach.

Let's instead hook this into our job engine, and try to activate
the service again when some other units are stopped. If all installed
jobs have been run yet we're still seeing the conflict or the manually
selected timeout is reached, fail the socket as before.

src/basic/unit-def.c

@@ -250,6 +250,7 @@ static const char* const socket_state_table[_SOCKET_STATE_MAX] = {
         [SOCKET_START_CHOWN]      = "start-chown",
         [SOCKET_START_POST]       = "start-post",
         [SOCKET_LISTENING]        = "listening",
+        [SOCKET_DEFERRED]         = "deferred",
         [SOCKET_RUNNING]          = "running",
         [SOCKET_STOP_PRE]         = "stop-pre",
         [SOCKET_STOP_PRE_SIGTERM] = "stop-pre-sigterm",

src/basic/unit-def.h

@@ -169,6 +169,7 @@ typedef enum SocketState {
         SOCKET_START_CHOWN,
         SOCKET_START_POST,
         SOCKET_LISTENING,
+        SOCKET_DEFERRED,
         SOCKET_RUNNING,
         SOCKET_STOP_PRE,
         SOCKET_STOP_PRE_SIGTERM,