[PATCH 1/3] Do not trigger user disconnect at g_at_shutdown

[Zhenhua Zhang]

Hi,

I rebased my patches for modem emulator and did some updates:

1. Patch 1/3 is the same as 4/4 sent on Feb 9th.
2. Patch 2/3 is updated according to Denis's comments. So we only have 
one full list at all.
3. Patch 3/3 is to fetch data from full list and write them into non 
blocking IO.

Please review them.

Thanks.
Zhenhua