env: fix env var autocompletion

commit 560d424b "env: re-add
support for auto-completion" fell short of its description -
the 'used' logic in hmatch_r was reversed - 'used' is 0 if
the hash table entry is not used, or -1 if deleted.  This
patch makes hmatch_r actually match on valid ('used') entries,
instead of skipping them and failing to match anything.

typing 'printenv tft' and hitting 'tab' now displays valid
choices for variable names.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Tested-by: Mike Frysinger <vapier@gentoo.org>