common: Add replacing fork/exec() to posix_spawn()

common: Add replacing fork/exec() to posix_spawn()

As applying unified-system-service,
there was an issue about fork/exec operation in the thread.
Actually, forking() in the thread is undefined behavior.
Thus if the storaged started as a unified-system-service,
fork operation should be replaced to posix_spawn() which is
safer method.
It will ignore parents(thread) signal setting and operated.

Change-Id: I9630335e6c6f0cf5dcf8705066181d6b0b30e305
Signed-off-by: Yunhee Seo <yuni.seo@samsung.com>