Add support for Silverlight

Add support for Silverlight exploitation. [SeeRM #8705]
Fix improper use of expand_path

I don't even.
Fix extraneous use of which and cmdsub

I don't even.
Anchor the scheme
Be more specific

Perl!
Land #2695, @wchen-r7's support to detect silverlight
Land #2699, @wvu fix for Linux download_exec post module
Fix fail_with on Exploit::Remote::HttpClient
fix fail_with usage on Exploit::Remote::MSSQL_SQLI
Land #2702, uninit const fix for HttpClient
Land #2703, uninit const fix for MSSQL_SQLI