Changes suggested by Pierre Humblet.

Changes suggested by Pierre Humblet.
* environ.cc (NL): New macro.
(conv_envvars): Use NL macro to fill in name and namelen.
(spenv::namelen): New field.
(spenvs): Use NL to fill in name and namelen.
(spenv::retrieve): Eliminate length argument.  Instead, use namelen throughout.
(build_env): Don't calculate length of initial FOO= part of environment.
Accommodate spenv::retrive argument change.