io: define SeekStart, SeekCurrent, SeekEnd constants for use with Seeker

[minux]

see issue #6873, and https://golang.org/cl/36810043/.

> We already have these in pkg/os, also with this TERRIBLE_CASE. Let's stop the
bleeding.
> In Go 2 we could move them and fix the case or maybe even change the signature to
not be so C-like.

[rsc]

Comment 1:

Labels changed: added repo-main.

[nightlyone]

Comment 2:

The actual use cases for seek are more along the lines of skipping bytes in a stream,
determining the size of a stream, rewinding a stream to the beginning, going to the end
and start using a io.Reader or io.Writer from a specific position. 
None of these require actual random access and thus still can be supported by compressed
streams.

[rsc]

Comment 3:

Adding Release=None to all Priority=Someday bugs.

Labels changed: added release-none.

[extemporalgenome]

Could we just duplicate those three "whence" consts into io, with properly cased names? (i.e. io.SeekEnd)

@nightlyone Size() int64 is a method that shows up occasionally, and its semantics are consistent across implementations. Instead of having separate methods for rewind and such, why not just use the existing interface, but have no-random-access implementations return errors on seeks they don't support?