portageq: clarify meaning of uses_root

portageq: clarify meaning of uses_root

o s/uses_root/uses_eroot/g -- This name was chosen
  before its meaning was changed; update it to reflect its purporse.

o Fix a left-over "<root>" arg-doc that never got updated to "<eroot>"

o If a provided eroot argument was shorter than the ${EPREFIX}, portageq
  was truncating it down to nothing and activating relative root,
  which means this became equivalent to an explicit argument of
  "${PWD}/${EPREFIX}/".  This is obviously wrong; and a simple case
  of failing to sanity-check inputs from an interface that changed in a
  backward-incompatible way.

  There's no provision in portageq to query EROOT's that don't end in
  ${EPREFIX}; so if the user makes such a request, instead of doing
  something pathological, dump a helpful error message and bail with
  a nonzero exit code.

Signed-off-by: Gregory M. Turner <gmturner007@ameritech.net>