Periodic commands

Hi Takasu,

This pull request is more like question: "do you find it useful and want to include it into your program after some fixes and refactoring?" than a real request.
To GUI STRSVR and RTKANI I add a possibility to send periodic user commands to input streams (serial port and network).
![2](https://cloud.githubusercontent.com/assets/3624285/3354210/add8414e-fa9b-11e3-93e3-1ba56af989da.png)

I programmed it in such way, so code is only added (exclude a few refactored parts), and if there is no defined positive constant MAXPERCMD, the program look the same as original.

Now this feature is only available in GUI STRSVR and RTKANI, but if you like it, I think the better way to use it in CUI too is firstly add in /src/streamsvr.c something like that:
extern int strsvrstart_wperiodic(strsvr_t *svr, int *opts, int *strs, char **paths, strconv_t **conv, const char *cmd, 	int* percmdsperiods, char** percmds, const double *nmeapos) that add to svr periodic commands and their periods and start usual strsvrstart. And of course add the same to src/rtksvr.c.

p.s. it also include the changes of previous pull request with hex commands ability.