spawn: Expose spawn functions API.
* src/gpg-error.def.in: Update. * src/gpg-error.vers: Update. * src/gpg-error.h.in: Add declarations. * src/visibility.c: Add implementations. * src/visibility.h: Expose them.
- GnuPG-bug-id: T6249
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>