Improved safety when copying things into strings

Created "safe_" variations of 'strncpy()', 'strncat()',
and 'snprintf()', to ensure a truncated source string doesn't
leave the destination buffer without a NUL termination character.

Replaced all calls (in "tuxpaint.c" only, so far) to the standard
functions with calls to the new safer versions.

Replaced most calls to plain 'strcpy()', 'strcat()' and
'sprintf()' (which can cause buffer overruns) with the new functions.

docs/CHANGES.txt

@@ -34,6 +34,10 @@ $Id$
       (Ref: https://github.com/haikuports/haikuports/issues/3045)
       Gerasim Troeglazov <3dEyes@gmail.com>
 
+  * Misc
+    ----
+    * Improved safety when copying things into string buffers.
+
 2020.April.24 (0.9.24)
   * New tools
     ---------