Add a "PYENV-SHIM-SCRIPT" marker in 2nd line of every shim file

Rationale: argcomplete library and possibily other tools that have
to follow the "wrapper chain" need to identify pyenv shims to be able
to read the target executable. setuptools do the same with its stubs.

see https://github.com/kislyuk/argcomplete/issues/58#issuecomment-23205305