Restore r276252 mdoc fix

It was accidentally reverted in the elftoolchain update (r276371).

Sponsored by:	The FreeBSD Foundation
Rename DT_FEATURE_1 to DT_FEATURE

Track r270303:

  This provides a minor cleanup in elfdump; there are otherwise no
  consumers in the tree.  Old SUN documentation can be found for either
  variant, but GNU binutils switched to DT_FEATURE around 2000.

Sponsored by:	The FreeBSD Foundation