New public API gpgrt_nvc_* and gpgrt_nve_*

* src/name-value.c: New.
* tests/t-name-value.c: New.
* src/gpg-error.def.in: Add new functions.
* src/gpg-error.h.in: Ditto.
* src/gpg-error.vers: Ditto.
* src/visibility.c: Ditto.
* src/visibility.h: Ditto.

These new functions were originally added to GnuPG taken from current
GnuPG master. The license has been changed from (LGPL-3.0-or-later OR
GPL-2.0-or-later) to LGPL-2.1-or-later: Everything was written by me
or employees/contractors of g10 Code GmbH. The private key handling
has been removed because that would require Libgcrypt functions. Some
functions are changed.