This removes the ability for templates to add custom HTTP headers, which can
easily be re-added if someone needs it. Thanks to asak for repeatedly reviewing
this patch and helping to iron out the quirks.
WSGI applications are not supposed to refer to sys.stdin. In af5aceab19f4,
hgweb and hgwebdir were fixed to pass interactive=False to their ui()'s, but
sys.stdin.isatty() was still called by the ui objects. This change makes sure
only the ui.fixconfig() method will call ui.isatty() (by making the
ui._readline() method, which is currently only called from ui.prompt(),
private). ui.fixconfig() is changed to let config files override the initial
interactivity setting, but not check isatty() if interactive=False was
specified in the creation of the ui.
