SourceLayout: shuffle HttpStatusLine into http/libsquid-http.la

SourceLayout: shuffle HttpStatusLine into http/libsquid-http.la

* moves HttpStatusLine.* to http/StatusLine.*

* renames HttpStatusLine to Http::StatusLine

* renames httpStatusLine*() functions as members of Http::StatusLine

* shuffles StatusCode string conversion function into http/StatusCode

* makes reason parameter of StatusLine::set() function optional.
  There is no logic change involved but callers now no longer need to
  set it to the status code string explicitly, nor need to set it to NULL
  explicitly unless intending to replace an existing status string.

* adds const-correctness and documentation to StatusLine symbols.