Initialize wxCSConv immediately instead of deferring it.

Deferred initialization code was not MT-safe and just wasn't that useful
anyhow because it is rare to create a wxCSConv object and not use it
afterwards.

Remove the deferred initialization logic and create the real conversion used
by wxCSConv immediately in its ctor.

Also improve the comments by clearly explaining the possible values of
wxCSConv::m_name and m_encoding.

Closes #12630.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66119 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775