Add most relevant vg_attr fields as lvm2app 'get' functions.

Add most relevant vg_attr fields as lvm2app 'get' functions.

Of the vgs field vg_attr, a few of the most likely to be used attributes
are clustered, exported, and partial.  This patch adds the following 3
functions:
uint64_t lvm_vg_is_clustered(const vg_t vg)
uint64_t lvm_vg_is_exported(const vg_t vg)
uint64_t lvm_vg_is_partial(const vg_t vg)