Fix false interpretation of vsn strings

rebar used to mistakenly report plain version strings like
{vsn, "1.0.0"} as follows:
DEBUG: vcs_vsn: Unknown VCS atom in vsn field: "1.0.0"

Properly distinguish between plain version strings and real
unknown version terms.

While at it, rename a variable in vcs_vsn/3 to be non-misleading.