* gpgme.def: Add gpgme_err_code_from_syserror and gpgme_err_set_errno.
* libgpgme.vers: Likewise.
* gpgme.h.in (gpgme_error_from_errno): Fix return type to
gpgme_error_t.
(gpgme_err_code_from_syserror, gpgme_err_set_errno): New prototype.
(gpgme_error_from_syserror): New inline function (why are
gpgme_err_make_from_errno and gpgme_error_from_errno not inline
functions?).
* error.c (gpgme_error_from_errno): Fix return type to gpgme_error_t.
(gpgme_err_set_errno, gpgme_err_code_from_syserror): New functions.
