VARNAMES bjam rule doesn't work for global module

[markovitch]

VARNAMES bjam rule always returns empty list for the
global module. 
This is due to the fact that builtin_varnames relies
upon module_t::variables struct member being not NULL
if there are variables in the module, which is wrong
for the "currently running module" (i.e.
frame->module->variables is _always_ NULL). This is due
to the way enter_module() and var_hash_swap() work.
Since builtin_varnames() always runs in the global
module context, global module's 'variables' member is
always NULL when builtin_varnames() is called.

The proposed patch fixes this bug.

[René Rivera]

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user_id=33595

Applied... Thanks for the patch :-)