dict: add dict_set_dynstr_with_alloc

There is an overwhelming no. of instances of the following pattern in
glusterd module.

    ...

    char *dynstr = gf_strdup (str);
    if (!dynstr)
       goto err;
    ret = dict_set_dynstr (dict, key, dynstr);
    if (ret)
       goto err;

    ...

With this changes it would look as below,

   ret = dict_set_dynstr_with_alloc (dict, key, str);
   if (ret)
       goto err;

Change-Id: I6a47b1cbab4834badadc48c56d0b5c8c06c6dd4d
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/7379
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>