busctl: Add introspect support for methods with same name but different signature

busctl: Add introspect support for methods with same name but different signature

D-Bus interfaces can have multiple methods with the same name, as long
as they have different arguments (signature). Currently busctl can call
those methods but when introspecting the interface it just displays
"Duplicate method"

This PR fixes the behavior, by also adding the signature to the hash for
the members set.

Before this patch:

$ busctl introspect org.asamk.Signal /org/asamk/Signal
Invalid introspection data: duplicate method 'sendMessage' on interface 'org.asamk.Signal'.

After this patch:

$ busctl introspect org.asamk.Signal /org/asamk/Signal
NAME                                TYPE      SIGNATURE RESULT/VALUE FLAGS
org.asamk.Signal                    interface -         -            -
.sendMessage                        method    as        x            -
.sendMessage                        method    s         x            -

Calling the methods already works as expected, as the user must specify
the signature explicitely:
busctl --user call org.asamk.Signal /org/asamk/Signal org.asamk.Signal sendMessage "as" 2 foo bar
busctl --user call org.asamk.Signal /org/asamk/Signal org.asamk.Signal sendMessage "s" foo

$ busctl --xml introspect org.asamk.Signal /org/asamk/Signal
<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN" "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
<node name="/org/asamk/Signal">
 <interface name="org.asamk.Signal">
  <method name="sendMessage" >
   <arg type="as" direction="in"/>
   <arg type="x" direction="out"/>
  </method>
  <method name="sendMessage" >
   <arg type="s" direction="in"/>
   <arg type="x" direction="out"/>
  </method>
 <interface name="org.freedesktop.DBus.Introspectable">
  <method name="Introspect">
   <arg type="s" direction="out"/>
  </method>
 </interface>
 <interface name="org.freedesktop.DBus.Peer">
  <method name="Ping">
  </method>
 </interface>
</node>