Add bool to rootcheck and syscheck to skip NFS mounted partitions

This is only implemented for Linux and FreeBSD as they are the only OS's I can track down decent documentation on how to accomplish this.  If this variable is set to "yes" and the feature isn't availble, a verbose error message is output to the OSSEC log file and the check returns false so the calling function will scan the directory as normal.

Need to document the change in ossec-docs, but basically adds a

    <skip_nfs>no</skip_nfs>

To the rootcheck and syscheck configurations.  To disable scanning of NFS shares:

    <skip_nfs>yes</skip_nfs>

Comments?
Do we want to use GCC os selection or the Makefile selection we use in other locations like: https://github.com/ossec/ossec-hids/blob/e443fee9bad8c5880e5070a4a0dfa4955970135c/src/rootcheck/check_rc_sys.c#L361 

```
            #ifdef SOLARIS
            if(strncmp(dir_name, "/boot", strlen("/boot")) != 0)
            {
                notify_rk(ALERT_ROOTKIT_FOUND, op_msg);
                _sys_errors++;
            }
            #elif defined(Darwin) || defined(FreeBSD)
            if(strncmp(dir_name, "/dev", strlen("/dev")) != 0)
            {
                notify_rk(ALERT_ROOTKIT_FOUND, op_msg);
                _sys_errors++;
            }
            #else
            notify_rk(ALERT_ROOTKIT_FOUND, op_msg);

            _sys_errors++;
            #endif
```
To be honest I am fine with this method, but I just want to make sure we do things the same way.