nsfAPI.h

New feature: added support for return value checker for nsf::proc
One can now specify "-returns ...." in nsf::proc commands the same way
as in nx/XOTcl2 methods.

nsf::proc ... /procName/ /arguments/ ?-returns /valueconstraint/? /body/

When return value checkers are defined, these are returned by the
usual introspection interface.

% nsf::proc ::foo {-a:integer} -returns integer {return 1}

% nsf::cmd::info definition ::foo
::nsf::proc ::foo -a:integer -returns integer {return 1}

% nsf::cmd::info returns ::foo
integer

The regression test was extended to cover these cases.