backup service: fall back to /sbin/bu

Change-Id: I46c67a401da8de92d7ef81ad40c565bb1d2ef93b
libsysutils: fix null pointer and memory leak issue

In SocketClient::quoteArg function
1. Fix potential null pointer accessing issue
2. Fix potential memory leak introduced by realloc fail

Change-Id: I1ca0f9089290d43452e9a71428244545f4ed866b
Signed-off-by: Hong-Mei Li <a21834@motorola.com>