Add ErrnoName and SignalName to get errno and signal name strings from
syscall.Errno and syscall.Signal values, respectively.

This repurposes the errors and signals vars (because they are not used
within x/sys/unix currently) and turns them into slices of struct,
containing errno/signal number, name and description. ErrnoName and
SignalName can then be trivially implemented using sort.Search.

Renaming errors to errorList additionaly allows to avoid package aliases
for the errors package.

Fixes golang/go#25134

Change-Id: Ie195872793f44c437f0f175ccfaa13a2546338c5
Reviewed-on: https://go-review.googlesource.com/110875
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>