Make the BSD patch WARNS=6 clean

Make the BSD patch WARNS=6 clean
- constify string operations if possible
- rename some variables to avoid namespace clashes
- don't cast the signal functions to sig_t), it's unnecessary

Submitted-by: Sascha Wildner <saw@online.de>
Instead of i_ptr[++iline][0] = '\0', I've used a special empty char array
instead. I'm not sure if i_ptr[++iline] points to a valid memory location
under all circumstances and this is a safer bet.