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