add stpncpy_chk builtin
disable fortify source
do not include config.h anymore

It has been empty for nearly all systems. People who used to put stuff
in config.h can still create a config.h on their own and inject -include
(gcc) or /FI (msvc) CFLAGS.
change input decoder selection

Selecting the decoder is a separate step now, so the error handling for
invalid encoding names can be moved out of input.c
move utf32 declaration to input.h to make it independent of the unicode utility functions
introduce UTF32 EOF char