MINOR: cfgcond: support negating conditional expressions

Now preceeding a config condition term with "!" will simply negate it.
Example:

   .if !feature(OPENSSL)
       .alert "SSL support is mandatory"
   .endif

doc/configuration.txt

@@ -811,6 +811,8 @@ The conditions are currently limited to:
   - the integer zero ('0'), always returns "false"
   - a non-nul integer (e.g. '1'), always returns "true".
   - a predicate optionally followed by argument(s) in parenthesis.
+  - a question mark ('!') preceeding any of the non-empty elements above, and
+    which will negate its status.
 
 The list of currently supported predicates is the following:
 
@@ -856,6 +858,10 @@ Example:
        profiling.memory on
    .endif
 
+   .if !feature(OPENSSL)
+       .alert "SSL support is mandatory"
+   .endif
+
 Four other directives are provided to report some status:
 
   - .diag "message"    : emit this message only when in diagnostic mode (-dD)

include/haproxy/cfgcond-t.h

@@ -71,6 +71,7 @@ struct cond_pred_kw {
 struct cfg_cond_term {
 	enum cfg_cond_term_type type; // CCTT_*
 	struct arg *args;             // arguments for predicates
+	int neg;                      // 0: direct result; 1: negate
 	union {
 		const struct cond_pred_kw *pred; // predicate (function)
 	};

src/cfgcond.c

@@ -65,6 +65,7 @@ int cfg_parse_cond_term(const char **text, struct cfg_cond_term *term, char **er
 
 	term->type = CCTT_NONE;
 	term->args = NULL;
+	term->neg  = 0;
 
 	while (*in == ' ' || *in == '\t')
 		in++;
@@ -72,6 +73,12 @@ int cfg_parse_cond_term(const char **text, struct cfg_cond_term *term, char **er
 	if (!*in) /* empty term does not parse */
 		return 0;
 
+	/* !<term> negates the term. White spaces permitted */
+	while (*in == '!') {
+		term->neg = !term->neg;
+		do { in++; } while (*in == ' ' || *in == '\t');
+	}
+
 	val = strtol(in, &end, 0);
 	if (end != in) {
 		term->type = val ? CCTT_TRUE : CCTT_FALSE;
@@ -173,6 +180,9 @@ int cfg_eval_cond_term(const struct cfg_cond_term *term, char **err)
 	else {
 		memprintf(err, "internal error: unhandled condition term type %d", (int)term->type);
 	}
+
+	if (ret >= 0 && term->neg)
+		ret = !ret;
 	return ret;
 }