example-sim800: update
at-unix: fix null pointer dereference
sim800: update for new AT api
example-sim800: fix printing of modem time
sim800: add gettime()
sim800_op_gettime: fix isdst on nuttx
modem/sim800: ops should be static
modem/sim800: offset is unused
at.h: mark at_command() with __attribute__ (printf)
sim800: add settime()
example-sim800: add settime() test
cellular.h: include network ops in ops, ffs
modem/sim800: add creg(), rssi()
example-sim800: add creg, rssi test