(PUP-1389) Ensure wide strings have double null terminators

It seems that calling String#encode('UTF-16LE') sometimes adds only a single
NULL terminator in both ruby 1.9 and 2.0 on Windows. However, a wide string
is supposed to have two NULL terminators.

For reasons unknown, versions prior to 2012 seem to accept wide strings with
only a single NULL terminator, but in 2012 and 2012R2 the string is often times
contains a non-zero high byte, which causes Windows to read past the single
terminator, and attempt to access a path that does not exist.
(PUP-1389) ReplaceFile to FFI / move wide_string

Puppet::Util::Windows::File.replace_file now calls FFI to
replace files in place on Windows.
Puppet::Util::Windows::File.wide_string has now been moved to
Puppet::Util::Windows::String.wide_string.

This moves wide_string to lib/puppet/ubilt/windows/string.rb,
adds specs concerning different encodings.

In addition, we've removed Windows::API::WideString usages from
everywhere it was found as it is known to cause issues with Windows 2012.
Puppet::Util::Windows::Process.lookup_privilege_value in particular was
changed to use LookupPrivilegeValueA (ANSI) as we control the privilege name
being passed to it.

Paired with Josh Cooper <josh@puppetlabs.com>
(PUP-1389) Simplify logic for appending extra NULL

This commit simplifies the process for appending a NULL terminator to
the wide string.
Merge branch 'pr/2261' into stable

* pr/2261:
  (PUP-1389) Simplify logic for appending extra NULL
  (PUP-1389) ReplaceFile to FFI / move wide_string
  (PUP-1389) Ensure wide strings have double null terminators