add bfmt.

src/libbio/bfmt.c

diff --git a/src/libbio/bfmt.c b/src/libbio/bfmt.c
new file mode 100644
index 0000000..f1b6f5a
--- /dev/null
+++ b/src/libbio/bfmt.c
@@ -0,0 +1,37 @@
+#include "lib9.h"
+#include <bio.h>
+
+static int
+_Bfmtflush(Fmt *f)
+{
+	Biobuf *b;
+
+	b = f->farg;
+	b->ocount = f->to - f->stop;
+	if(Bflush(b) < 0)
+		return 0;
+	f->to = b->ebuf+b->ocount;
+	return 1;
+}
+
+int
+Bfmtinit(Fmt *f, Biobuf *b)
+{
+	if(b->state != Bwactive)
+		return -1;
+	memset(f, 0, sizeof *f);
+	f->farg = b;
+	f->start = b->bbuf;
+	f->to = b->ebuf+b->ocount;
+	f->stop = b->ebuf;
+	f->flush = _Bfmtflush;
+	return 0;
+}
+
+int
+Bfmtflush(Fmt *f)
+{
+	if(_Bfmtflush(f) <= 0)
+		return -1;
+	return f->nfmt;
+}