lib9: add write function that withstands interrupts

src/lib9/mkfile

@@ -157,6 +157,7 @@ LIB9OFILES=\
 	unsharp.$O\
 	wait.$O\
 	waitpid.$O\
+	write.$O\
 
 OFILES=\
 	$LIB9OFILES\

src/lib9/write.c

@@ -0,0 +1,23 @@
+#include <u.h>
+#define NOPLAN9DEFINES
+#include <libc.h>
+
+long
+p9write(int f, void *av, long n)
+{
+	char *a;
+	long m, t;
+
+	a = av;
+	t = 0;
+	while(t < n){
+		m = write(f, a+t, n-t);
+		if(m <= 0){
+			if(t == 0)
+				return m;
+			break;
+		}
+		t += m;
+	}
+	return t;
+}