spawn: Support the use case in libassuan where PGMNAME==NULL.
* src/spawn-posix.c (my_exec): When PGMNAME==NULL, just fork. (_gpgrt_process_spawn): Support PGMNAME==NULL use case. * src/spawn-w32.c (_gpgrt_process_spawn): Return GPG_ERR_INV_ARG, when PGMNAME==NULL.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>