Allow 'make buildenv' to default to the caller's shell by using SHELL.

Allow 'make buildenv' to default to the caller's shell by using SHELL.

Also pass BUILDENV=1 into the sub-shell to allow modifying PS1 in .profile such
as:
  if [ -n "${BUILDENV}" ]; then
          PS1="(buildenv) ${PS1}"
  fi

SHELL defaults to 'sh' in share/mk/sys.mk, but is typically passed down by
the shell invoking make as well.  Rather than forcing all 'buildenv' users
to use plain /bin/sh, let them use their favorite shell.

MFC after:	2 weeks
Sponsored by:	EMC / Isilon Storage Division
Discussed with:	imp