rename libc to libjehanne; improve libposix

This is to avoid conflicts between standards' assumptions and Jehanne's choices

sys/src/lib/jehanne/port/strcat.c

@@ -0,0 +1,19 @@
+/*
+ * This file is part of the UCB release of Plan 9. It is subject to the license
+ * terms in the LICENSE file found in the top-level directory of this
+ * distribution and at http://akaros.cs.berkeley.edu/files/Plan9License. No
+ * part of the UCB release of Plan 9, including this file, may be copied,
+ * modified, propagated, or distributed except according to the terms contained
+ * in the LICENSE file.
+ */
+
+#include <u.h>
+#include <libc.h>
+
+char*
+jehanne_strcat(char *s1, const char *s2)
+{
+
+	jehanne_strcpy(jehanne_strchr(s1, 0), s2);
+	return s1;
+}