std: move errno to become an nonexhaustive enum

[andrewrk]

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.

[daurnimator]

See also #4233