Const fixing

Const fixing

Fixing some const warnings - with API change in:

int vg_extend(struct volume_group *vg, int pv_count, const char *const *pv_names,

Change is needed - as lvm2api expects const behaviour here.
So vg_extend() is doing local strdup for unescaping.

skip_dev_dir return const char* from const char* vg_name.

Rest of the patch is cleanup of related warnings.

Also using dm_report_filed_string() API change to simplify
casting in _string_disp and _lvname_disp.