os/exec: fix test hang
Currently the test closes random files descriptors,
which leads to hang (in particular if netpoll fd is closed).
Try to open only fd 3, since the parent process expects it to be fd 3 anyway.
Fixes issue 5571.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/9778048
runtime: allocate internal symbol table eagerly
we need it for GC anyway.

R=golang-dev, khr, dave, khr
CC=golang-dev
https://codereview.appspot.com/9728044
Merge branch 'master' into gitfix
runtime: allocate page table lazily
This removes the 256MB memory allocation at startup,
which conflicts with ulimit.
Also will allow to eliminate an unnecessary memory dereference in GC,
because the page table is usually mapped at known address.
Update issue 5049.
Update issue 5236.

R=golang-dev, khr, r, khr, rsc
CC=golang-dev
https://codereview.appspot.com/9791044
runtime: make mheap statically allocated again
This depends on: 9791044: runtime: allocate page table lazily
Once page table is moved out of heap, the heap becomes small.
This removes unnecessary dereferences during heap access.
No logical changes.

R=golang-dev, khr
CC=golang-dev
https://codereview.appspot.com/9802043
Merge branch 'master' into gitfix