env: add -v/--debug option

env: add -v/--debug option

Prints verbose information about each step:

  $ env -v -uFOO  -C /tmp BAR=BAZ date -u
  env: unset:    FOO
  env: setenv:   BAR=BAZ
  env: chdir:    '/tmp'
  env: executing: date
  env:    arg[0]= ‘date’
  env:    arg[1]= ‘-u’
  Sun Apr 22 08:52:30 UTC 2018

Inspired by FreeBSD's env(1).

* src/env.c (usage): Mention new option.
  (main): Print debug information if requested.
* NEWS: Mention new option.
* doc/coreutils.texi (env invocation): Mention -v/--debug.