syscall: add PtraceSyscall(pid int)

[rsc]

Requested on golang-nuts: add PtraceSyscall(pid int).
Would match the other Ptrace functions.

[minux]

Comment 1:

For the record, original discussion on golang-nuts:
http://groups.google.com/group/golang-nuts/browse_thread/thread/2c94f0cc81943d35

[davecheney]

Comment 2:

http://golang.org/cl/6353051 contains a possible solution.

[davecheney]

Comment 3:

This issue was closed by revision dd79b33.

Status changed to Fixed.