common,w32: Make use of default_errsource in exechelp.
* common/exechelp-posix.c (my_error_from_syserror, my_error): New. Use them instead of gpg_error and gpg_error_from_syserror.
- Fixes-commit: 96c7901ec1c79be732570811223d3ea54875abfe
- Signed-off-by: Justus Winter <justus@g10code.com>