net: disable TestDialMultiFDLeak

It fails about 25% of the time on OS X.
I don't know what it's trying to do.
Created issue 8764 to correct this, but for now disable.

LGTM=bradfitz, mikioh.mikioh
R=bradfitz, mikioh.mikioh
CC=golang-codereviews
https://codereview.appspot.com/144070044
os: avoid error result when dir is removed out from under RemoveAll

Fixes issue 7776.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews, r
https://codereview.appspot.com/145860043
runtime: delete panicstring; move its checks into gopanic

In Go 1.3 the runtime called panicstring to report errors like
divide by zero or memory faults. Now we call panic (gopanic)
with pre-allocated error values. That new path is missing the
checking that panicstring did, so add it there.

The only call to panicstring left is in cnew, which is problematic
because if it fails, probably the heap is corrupt. In that case,
calling panicstring creates a new errorCString (no allocation there),
but then panic tries to print it, invoking errorCString.Error, which
does a string concatenation (allocating), which then dies.
Replace that one panicstring with a throw: cnew is for allocating
runtime data structures and should never ask for an inappropriate
amount of memory.

With panicstring gone, delete newErrorCString, errorCString.
While we're here, delete newErrorString, not called by anyone.
(It can't be: that would be C code calling Go code that might
block or grow the stack.)

Found while debugging a malloc corruption.
This resulted in 'panic during panic' instead of a more useful message.

LGTM=khr
R=khr
CC=golang-codereviews
https://codereview.appspot.com/138290045