Improve how the parser is generated

- use long variant of options for re2c in its makefile
- use `define` instead of magic numbers
- add some consts
- trailing `;` are now mandatory for conditions
- NULL bytes are no longer allowed in configuration file
- the parser shouldn't crash in the absence of trailing new line at the end of its configuration file

src/Makefile.frag

@@ -1,7 +1,7 @@
 $(srcdir)/sp_config_scanner.c: $(srcdir)/sp_config_scanner.re
 	if re2c -v |grep ' [23]\.' 2>/dev/null; then \
-		re2c -bc -o $@ $<; \
-		re2c --no-generation-date --no-version -bci -o $(srcdir)/sp_config_scanner.cached.c $<; \
+		re2c -W --conditions --bit-vectors --no-debug-info --output $@ $<; \
+		re2c -W --no-generation-date --bit-vectors --no-version --conditions --no-debug-info --output $(srcdir)/sp_config_scanner.cached.c $<; \
 	else \
 		cp $(srcdir)/sp_config_scanner.cached.c $@; \
 	fi;