Disallow more than one signal= specification in an injection expression

Specifying several signal= specifications in an injection expressions
makes no sense.

* filter_qualify.c (parse_inject_token): Disallow second
signal= specification.
* strace.1.in: Document it.
* tests/qual_inject-syntax.test: Check it.

filter_qualify.c

@@ -140,6 +140,8 @@ parse_inject_token(const char *const token, struct inject_opts *const fopts,
 		fopts->data.flags |= INJECT_F_RETVAL;
 	} else if (!fault_tokens_only
 		   && (val = STR_STRIP_PREFIX(token, "signal=")) != token) {
+		if (fopts->data.flags & INJECT_F_SIGNAL)
+			return false;
 		intval = sigstr_to_uint(val);
 		if (intval < 1 || intval > NSIG_BYTES * 8)
 			return false;

strace.1.in

@@ -654,8 +654,9 @@ An injection expression can contain only one
 .BR error =
 or
 .BR retval =
-specification.
-If an injection expression contains multiple
+specification, and only one
+.BR signal =
+specification.  If an injection expression contains multiple
 .BR when =
 specifications, the last one takes precedence.
 

tests/qual_inject-syntax.test

@@ -103,6 +103,8 @@ for arg in '' , ,, ,,, : :: ::: \! \!, \!: \
 	   chdir:retval=-1 \
 	   chdir:signal=0 \
 	   chdir:signal=129 \
+	   chdir:signal=1:signal=2 \
+	   chdir:signal=1:retval=0:signal=2 \
 	   chdir:retval=0:retval=1 \
 	   chdir:error=1:error=2 \
 	   chdir:retval=0:error=1 \