The set of errors forwarded by the os package varied with system and
was therefore non-portable.
Three helpers added for portable error checking: IsExist, IsNotExist, and IsPermission.
One or two more may need to come, but let's keep the set very small to discourage
thinking about errors that way.
R=mikioh.mikioh, gustavo, r, rsc
CC=golang-dev
https://golang.org/cl/5672047
All tests enabled by default passes except those in timeout_test.go.
For TestLookupPort, add an entry for "bootps" in /lib/ndb/common
(Plan 9 calls it "bootp"). I've sent out a patch to fix this.
R=paulzhol, rsc, mikioh.mikioh
CC=ality, golang-dev
https://golang.org/cl/4779041
