When system calls indicate an error they return -1, not some arbitrary

value < 0.  errno is only updated in this case.  Change all (most?)
callers of syscalls to follow this better, and let's see if this strictness
helps us in the future.

libexec/tradcpp/files.c

@@ -336,7 +336,7 @@ file_tryopen(const char *file)
 	/* XXX check for non-regular files */
 
 	fd = open(file, O_RDONLY);
-	if (fd < 0) {
+	if (fd == -1) {
 		if (errno != ENOENT && errno != ENOTDIR) {
 			complain(NULL, "%s: %s", file, strerror(errno));
 		}