Suggest io.SeekXXX instead of os.SEEK_XXX

[dsnet]

We added the io.SeekXxx constants in Go1.7, we should suggest those constants when people try to use the os.SEEK_XXX constants.

[pravj]

@dsnet,

I would like to work on this.
If I'm allowed to do so, can you please hint me to start with this?

[dmitshur]

FWIW, there's overlap with staticcheck, this check already exists there as:

SA1019 – Using a deprecated function, variable, constant or field

/cc @dominikh FYI.

[andybons]

Closing due to #302 (comment)