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nusb/ptp: transaction IDs are sequences of numbers starting with 0 (thanks unobe)

According to 'Digital Camera connectivity solutions using the picture
transfer protocol' [1] Section 2.3: "the PTP is specified using the
transaction model.  A transaction is composed of a request operation,
followed by an optional data transfer and a response.  Each
transaction has an identifier (TransactionID) that is session unique
and comprise of a 32 bit unsigned number.  The transaction IDs are a
sequence of numbers starting with 0x00000000 (for the SessionOpen
transaction) and increasing with every following operation."

[1] https://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/f2012/jmv87/site/files/PTP%20Protocol.pdf

--- a/sys/man/4/nusb

+++ b/sys/man/4/nusb

@@ -194,6 +194,10 @@

 The

 .B ctl

 file supplies the device geometry and partitions when read.

+.I Ptp

+configures and exposes a camera's (and often a mobile

+device's) storage to allow transferring data via the

+Picture Transfer Protocol (PTP).

 .SS Ethernet

 .I Ether

 handles USB ethernet devices. The file system provided is

--- a/sys/src/cmd/nusb/ptp/ptp.c

+++ b/sys/src/cmd/nusb/ptp/ptp.c

@@ -75,9 +75,8 @@

 static int debug = 0;

 static ulong time0;

-static int maxpacket = 64;

-static int sessionId = 1;

-static int transId = 1;

+static int sessionId;

+static int transId;

 static Node **nodes;

 static int nnodes;

@@ -556,7 +555,7 @@

 		}

 		/*

-		 * another proc migh'v come in and done it for us,

+		 * another proc might've come in and done it for us,

 		 * so check the cache again.

 		 */

 		if(y = cachednode(path, &f))

@@ -1048,6 +1047,7 @@

 	sendp(iochan, ioproc());

 	sessionId = getpid();

+	transId   = 0;

 	if(ptprpc(nil, OpenSession, 1, sessionId) < 0)

 		sysfatal("open session: %r");