all: fix typos

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/8896045
cmd/dist: fix line numbers in goc2c generated files

We have to reset the global lineno variable before
processing each file otherwise line numbers will be
offset by the number of lines in the previous file.

The following examples are from the beginning of the
ztime_linux_amd64.c file which is generated from
time.goc in the runtime package.

Before:
    #line 2483 "/home/apm/src/go/src/pkg/runtime/time.goc"
    static Timers timers;
    static void addtimer ( Timer* ) ;
    void
    time·Sleep(int64 ns)
    {
    #line 2492 "/home/apm/src/go/src/pkg/runtime/time.goc"

After:
    #line 16 "/home/apm/src/go/src/pkg/runtime/time.goc"
    static Timers timers;
    static void addtimer ( Timer* ) ;
    void
    time·Sleep(int64 ns)
    {
    #line 25 "/home/apm/src/go/src/pkg/runtime/time.goc"

R=golang-dev, minux.ma, iant, r, adg
CC=golang-dev
https://codereview.appspot.com/8653045
net: add missing File method document

R=golang-dev, bradfitz, r
CC=golang-dev
https://codereview.appspot.com/8698049
cmd/5g, cmd/6g, cmd/8g: more nil ptr to large struct checks

R=r, ken, khr, daniel.morsing
CC=dsymonds, golang-dev, rickyz
https://codereview.appspot.com/8925043
cmd/vet: clean up the test
- clean up the notypes version of the test so it's quiet
- change the package in the buildtag test to avoid confusing godoc

R=golang-dev, minux.ma, bradfitz
CC=golang-dev
https://codereview.appspot.com/8671049