Make strace correctly handle SIGTRAP produced by e.g.

[Roland McGrath]

> Two months ago I said that I'd like to see reintroduced one change (which
> was lost with big revert commit) made by Denys: "Replace many more bare
> ptrace calls with calls to wrappers which do proper error-checking and
> set tcp->ptrace_errno."  Unfortunately, I failed to allocate time required
> to implement and test it.  Anyway, current HEAD is much better than last
> release made more than a year ago.

Agreed.  Let's not have any more delay.  We can do all those things after
the release.