fwknop uses unsafe _snprintf() on Windows

[Pierre Pronchery]

In common/common.h, line 93:

 87 /* Some hoops for accommodating Windows
 88 */
 89 #ifdef WIN32
 90   #include <io.h>
 91   #define strcasecmp    _stricmp
 92   #define strncasecmp   _strnicmp
 93   #define snprintf              _snprintf

and in lib/fko_common.h, line 66:

 62 #ifdef WIN32
 63   #include <io.h>
 64   #define strcasecmp  _stricmp
 65   #define strncasecmp _strnicmp
 66   #define snprintf    _snprintf

Windows builds of fwknop are set to use _snprintf() when snprintf() is normally used. This is discouraged, and even dangerous, since the implementation of _snprintf() on Windows is not behaving as expected: it does not automatically terminate strings upon overflows.
See https://msdn.microsoft.com/en-us/library/2ts7cx93.aspx for a complete explanation.
Perhaps unbeknownst to this fact, as it happens many snprintf() calls in the code are fortunately performed this way:

char buf[BUF_SIZE] = {0}; //initialization with all-zeros apparently

snprintf(buf, sizeof(buf) - 1, "%s", input_string);

On UNIX this wastes the space for one character, since snprintf() will truncate the buffer at sizeof(buf) - 2 as expected. On Windows luckily even though the buffer was not truncated there, the buffer initialization will apparently guarantee the presence of a terminating character.
Now that Windows supports a C99-conformant implementation of snprintf(), it should be possible to remove this crude hack. In the meantime every snprintf() call should probably be reviewed for correctness there.