* added comment
* replaced "~0" with the appropriate macro
* resolved -Wconversion
* saved some space by modifying AP_Param::add_vector_suffix (writing
behind buffer could happen, if buffer_size is almost uint16_max but this
can surely be ignored)

further reading:
http://stackoverflow.com/questions/809227/is-it-safe-to-use-1-to-set-all-bits-to-true
-Wsign-conversion