Mycelium shared stack

[oooo-ps]

Experimental implementation for PurpleI2P#2245

[oooo-ps]

Implementation of the libi2pd/Transports.cpp appears unclear for me, but also at this point I think that going by the incorrect way: the eNTCP2V6MeshIdx should be or dual-stack or i2pd should provide mesh-only config option (instead of ygg* one)

Maybe closing, feel free to reuse.