Check setsockopt() RC and add more useful messages when mroute socket is busy

Fix Coverity Scan CID #60560, #60563

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>
smcroute.c: Fix potential negative array index write

Coverity Scan CID #61536

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>
Fix potential string not NUL terminated bug, Coverity CID #60558

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>