ape/stdio: make fopen() quasi threadsafe for python

python uses processes sharing memory. it requires at least fopen() to
be called by multiple threads at once so we introduce _IO_newfile()
which allocates the FILE structure slot under a lock.

sys/src/ape/lib/ap/stdio/_IO_newfile.c

@@ -0,0 +1,27 @@
+/*
+ * pANS stdio -- fopen
+ */
+#include "iolib.h"
+
+#define _PLAN9_SOURCE
+#include <lock.h>
+
+FILE *_IO_newfile(void)
+{
+	static FILE *fx=0;
+	static Lock fl;
+	FILE *f;
+	int i;
+
+	lock(&fl);
+	for(i=0; i<FOPEN_MAX; i++){
+		if(fx==0 || ++fx >= &_IO_stream[FOPEN_MAX]) fx=_IO_stream;
+		if(fx->state==CLOSED)
+			break;
+	}
+	f = fx;
+	unlock(&fl);
+	if(f->state!=CLOSED)
+		return NULL;
+	return f;
+}