Modbus feature 639 v1.5

This pull request adds support for Modbus protocol (rule keyword : modbus).

This pull request is an update of the Modbus feature 639 v1.4 PR to fix warnings.
Compilers think value can be used uninitialized:
<pre>
detect-engine-modbus.c: In function ‘DetectEngineInspectModbus’:
detect-engine-modbus.c:136:30: error: ‘value’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
             value   = (value >> offset) & 0x1;
                              ^
detect-engine-modbus.c:108:22: note: ‘value’ was declared here
     uint16_t offset, value, type = tx->type;
                      ^
cc1: all warnings being treated as errors
make[3]: *** [detect-engine-modbus.o] Error 1
</pre>