shell: introduce sdb user daemon for user shell

shell: introduce sdb user daemon for user shell

To create a shell session via sdb, SDBD open the ptmx and then a pts
device is created. Next, SDBD forks a shell who use the pts opened
previously. BUT, forked shell cannot use the pts because forked shell
has no privilege to read the pts.
So I introduce new sdb user daemon who is only for launching shell.
The sdb user daemon has low privilege, same as user shell, and has few
functions - open ptmx, pass opened ptmx fd to SDBD, and exec shell.

Change-Id: I2332419e4b186d4b18c281a554569735617e9f64
Signed-off-by: Sooyoung Ha <yoosah.ha@samsung.com>