common: Avoid warning about const char ** assignment.

* common/mkdir_p.c (gnupg_amkdir_p): Also strdup first item.  Return
an error on malloc failure.
(gnupg_mkdir_p): Fix type of dirs and tmp_dirs.

The code was correct but it inhibits type checking. Instead of
casting it seems easier to simply allocate also the the first item in
DIRS.

• Signed-off-by: Werner Koch <wk@gnupg.org>