put confstr.c with the other conf functions

src/conf/confstr.c

@@ -0,0 +1,17 @@
+#include <unistd.h>
+#include <stdio.h>
+#include <errno.h>
+
+size_t confstr(int name, char *buf, size_t len)
+{
+	const char *s = "";
+	if (!name) {
+		s = "/bin:/usr/bin";
+	} else if ((name&~4U)!=1 && name-_CS_POSIX_V6_ILP32_OFF32_CFLAGS>31U) {
+		errno = EINVAL;
+		return 0;
+	}
+	// snprintf is overkill but avoid wasting code size to implement
+	// this completely useless function and its truncation semantics
+	return snprintf(buf, len, "%s", s);
+}