fix buffer overflow for unknown mode numbers

to test, try /mode 1337 ; map ac_desert
replace textfilter functions with one-fits-all version

* replaces filtertext() with a version with a dozen of single filters
  that can be used in any combination
* defines classes of allowed characters for strings for various uses
  (see protocol.h line 153ff)
* now the allowed charset of, for example, player names is defined
  in one location only
* fixes issue #58