We already have a LICENSE file that covers the Zig Standard Library. We
no longer need to remind everyone that the license is MIT in every single
file.
Previously this was introduced to clarify the situation for a fork of
Zig that made Zig's LICENSE file harder to find, and replaced it with
their own license that required annual payments to their company.
However that fork now appears to be dead. So there is no need to
reinforce the copyright notice in every single file.
The primary purpose of this change is to eliminate one usage of
`usingnamespace` in the standard library - specifically the usage for
errno values in `std.os.linux`.
This is accomplished by truncating the `E` prefix from error values, and
making errno a proper enum.
A similar strategy can be used to eliminate some other `usingnamespace`
sites in the std lib.
