Escape @string so it can appear in a D-Bus address as the value
part of a key-value pair.
For instance, if @string is "/run/bus-for-:0",
this function would return "/run/bus-for-%3A0",
which could be used in a D-Bus address like
"unix:nonce-tcp:host=127.0.0.1,port=42,noncefile=/run/bus-for-%3A0".
Escape @string so it can appear in a D-Bus address as the value part of a key-value pair.
For instance, if @string is "/run/bus-for-:0", this function would return "/run/bus-for-%3A0", which could be used in a D-Bus address like "unix:nonce-tcp:host=127.0.0.1,port=42,noncefile=/run/bus-for-%3A0".