libc: introduce "jehanne_" namespace

With this commit all functions declared in libc.h have been renamed
with the "jehanne_" prefix. This is done for several reason:

- it removes conflicts during symbol resolution when linking
  standard C libraries like newlib or musl
- it allows programs depending on a standard C library to directly
  link to a library depending on our non standard libc (eg libsec).

To ease transiction two files are provided:

- sys/include/lib9.h that can be included instead of <libc.h> to use
  the old names (via a simple set of macros)
- sys/src/lib/c/lib9.c that can be compiled with a program where the
  macro provided by lib9.h are too dumb (see for example rc or grep).

In the kernel port/lib.h has been modified accordingly and some of
the functions it directly provides has been renamed too (eg malloc
in qmalloc.c and print in devcons.c).

sys/include/apw/stdlib.h

@@ -22,7 +22,7 @@
 #define EXIT_SUCCESS 0
 #define EXIT_FAILURE 1	// POSIX
 
-#define __POSIX_EXIT_PREFIX "posix exit: "
+#include <posix.h>	// to get __POSIX_EXIT_PREFIX
 
 typedef	struct {
 	int	quot;