IIOD: Don't set a signal handler on SIGSEGV

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
IIOD: Add proper error reporting

Previously, if the requested device / channel in a command was not
found, the interpreter was issuing a syntax error. This was not
accurate.

Now, the commands will return -ENODEV if the device is not found, or
-ENXIO if the device is found but the channel is not.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>