Change _p9strsig to return nil if tmp==nil.

Add atoi, atol, atoll.
These versions call strtol/strtoll with base==0.
The Unix versions use base==10.

src/lib9/atoll.c

@@ -0,0 +1,9 @@
+#include <u.h>
+#include <libc.h>
+
+vlong
+atoll(char *s)
+{
+	return strtoll(s, 0, 0);
+}
+