Use snprintf and strnstr

[sfe-SparkFro]

From @gigapod in #10 in response to extensive usage of sprintf and strstr, such as here:

SparkFun_u-blox_Cellular_Arduino_Library/src/sfe_ublox_cellular.cpp

Line 873 in d26ede0

sprintf(command, "%s=%d", UBX_CELL_REGISTRATION_STATUS, 2/*enable URC with location*/);

and here:

SparkFun_u-blox_Cellular_Arduino_Library/src/sfe_ublox_cellular.cpp

Line 321 in d26ede0

char *searchPtr = strstr(event, UBX_CELL_READ_SOCKET_URC);