core: Change new functions to return gpg_err_code_t.

* src/gpg-error.h.in (gpgrt_sentenv, gpgrt_mkdir, gpgrt_chdir): Change
return type to gpg_err_code_t.
* src/sysutils.c (_gpgrt_setenv): Implement that.
(_gpgrt_mkdir): Ditto.
(_gpgrt_chdir): Ditto.
* gpgscm/ffi.c (do_setenv, do_mkdir): Adjust for this change.

The functions are already a bit different and thus it is useful to let
them return a gpg-error style error code and don't let the user
implement that for each call.

• Signed-off-by: Werner Koch <wk@gnupg.org>