Improve the posix preprocessor.

This fixes token pasting, making it expand when
it should expand, and paste before expansion when
it should paste before expanding.

	#define CAT(a, b) a ## b
	#define BAR	3
	#define FOO	CAT(BAR, 3)
	FOO

now produces 33, while

	#define CAT(a, b) a ## b
	#define EOF	(-1)
	#define NOP(x)	x
	NOP(CAT(foo, EOF))
	CAT(,EOF)
	CAT(,)

produces

	fooEOF
	(-1)
	<empty>

respectively.

1 files changed, 49 insertions, 0 deletions

diff --git a/sys/src/cmd/cpp/test.expected b/sys/src/cmd/cpp/test.expected
new file mode 100644
index 000000000..bf4b3aa99
--- /dev/null
+++ b/sys/src/cmd/cpp/test.expected
@@ -0,0 +1,49 @@
+#line 1 "/usr/ori/src/cpp/test.c"
+
+
+
+x fooEOF y
+x EOFfoo y
+x(-1) y
+y foo x
+x foo y
+X y
+
+
+ foobar
+
+
+foo ## bar
+
+
+ ablahb
+
+
+
+ 33
+
+
+#line 32 "/usr/ori/src/cpp/test.c"
+
+
+
+
+ a,b
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ f(2 * (y+1)) + f(2 * (f(2 * (z[0])))) % f(2 * (0)) + t(1);
+ f(2 * (2+(3,4)- 0,1)) | f(2 * (~ 5)) & f(2 * (0,1))^ m(0,1);
+#line 55 "/usr/ori/src/cpp/test.c"