RPC, cosmetic: push down ReadHTTPStatus() calls into ReadHTTP() callers

ReadHTTPStatus() is currently overloaded:  In client mode, it properly parses
and receives an HTTP status line.  In server mode, it incorrectly parses the
HTTP request line as an HTTP status line.

This server mode bug has never mattered, because the RPC server never
cared about the URI (path) provided in the HTTP request.  That will change in
the future, so go ahead and begin fixing the problem.

This patch is cosmetic, and should result in NO behavior changes.

Further renames:
ReadHTTPHeader -> ReadHTTPHeaders
ReadHTTP -> ReadHTTPMessage
RPC: HTTP server uses its own ReadHTTPRequestLine()
rather than reusing ReadHTTPStatus() from the client mode.

The following additional HTTP request validations are added, both in line with
existing HTTP client practice:

1) HTTP method must be GET or POST.  Most clients use POST, some
   use GET.  Either way, this continues to work.
2) HTTP URI must start with "/" character.
   Normal URI is "/" (a 1-char string), so this is fine.
Bitcoin-Qt: prevent stuck/unusable debug window on exit

- when closing the client with an open debug window, that window could
  become stuck/unsuable (it was still shown wherea the main window was
  hidden already)
- fix this by hiding the debug window, when quitting the the client