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.