use xsnprintf instead of snprintf.

snprintf is not available on LynxOS, so I changed the calls to snprintf
to calls to xsnprintf, which should be strictly equivalent.

gdb/gdbserver/ChangeLog:

        * utils.c (xsnprintf): Make non-static.
        * server.h: Add xsnprintf declaration.
        * linux-low.c, nto-low.c, target.c, thread-db.c, tracepoint.c:
        replace calls to snprintf by calls to xsnprintf throughout.

gdb/gdbserver/utils.c

@@ -244,7 +244,7 @@ get_cell (void)
 /* Stdarg wrapper around vsnprintf.
    SIZE is the size of the buffer pointed to by STR.  */
 
-static int
+int
 xsnprintf (char *str, size_t size, const char *format, ...)
 {
   va_list args;