use io.SEEK_* constants instead of os.SEEK_* where an IO stream is seeked, leaving the os.SEEK_* constants only for os.lseek, as documented

Vilmos Nebehaj.

Vilmos Nebehaj.

Issue #12715: Add an optional symlinks argument to shutil functions (copyfile, copymode, copystat, copy, copy2).
When that parameter is true, symlinks aren't dereferenced and the operation
instead acts on the symlink itself (or creates one, if relevant).

Patch by Hynek Schlawack.