common: Fix a race condition in creating socketdir.
* common/homedir.c (_gnupg_socketdir_internal): Check return code of gnupg_mkdir and handle the case of GPG_ERR_EEXIST.
(cherry picked from commit 71840b57f48680b7555451a29026d9c6de4fe2bc)
- GnuPG-bug-id: T7332
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>