Conditional Config: add @warning "aaa" and @error "bbb"

As usual, this is mostly for configuration templates that you use for
multiple servers, that sort of things, eg.

@if !environment("ADMIN")
@error "Environment variable ADMIN is not set"
@endif

This also adds a change in conf.c so @define, @error and
@warning are skipped in @if blocks that evaluate to false
(that's obviously what everyone wants :D). So that fixes a
previous bug with @define in @if.

include/h.h

@@ -80,6 +80,7 @@ extern PreprocessorItem parse_preprocessor_item(char *start, char *end, const ch
 extern void preprocessor_cc_duplicate_list(ConditionalConfig *r, ConditionalConfig **out);
 extern void preprocessor_cc_free_level(ConditionalConfig **cc_list, int level);
 extern void preprocessor_cc_free_list(ConditionalConfig *cc);
+extern int preprocessor_resolve_if(ConditionalConfig *cc, PreprocessorPhase phase);
 extern void preprocessor_resolve_conditionals_ce(ConfigEntry **ce_list, PreprocessorPhase phase);
 extern void preprocessor_resolve_conditionals_all(PreprocessorPhase phase);
 extern void init_config_defines(void);

include/struct.h

@@ -1611,7 +1611,9 @@ typedef enum PreprocessorItem {
 	PREPROCESSOR_DEFINE		= 1,
 	PREPROCESSOR_IF			= 2,
 	PREPROCESSOR_ENDIF		= 3,
-	PREPROCESSOR_ELSE		= 4
+	PREPROCESSOR_ELSE		= 4,
+	PREPROCESSOR_USER_ERROR		= 5,
+	PREPROCESSOR_USER_WARNING	= 6
 } PreprocessorItem;
 
 typedef enum PreprocessorPhase {