lib9: import frand function from Plan 9

Fixes #15.

src/lib9/frand.c

@@ -0,0 +1,17 @@
+#include	<u.h>
+#include	<libc.h>
+
+#define	MASK	0x7fffffffL
+#define	NORM	(1.0/(1.0+MASK))
+
+double
+p9frand(void)
+{
+	double x;
+
+	do {
+		x = lrand() * NORM;
+		x = (x + lrand()) * NORM;
+	} while(x >= 1);
+	return x;
+}

src/lib9/mkfile

@@ -102,6 +102,7 @@ LIB9OFILES=\
 	execl.$O\
 	exitcode.$O\
 	fcallfmt.$O\
+	frand.$O\
 	get9root.$O\
 	getcallerpc-$OBJTYPE.$O\
 	getenv.$O\