MEM: Add the port proxies to the source tree

Port proxies are used to replace non-structural ports, and thus enable
all ports in the system to correspond to a structural entity.

The following replacements are going to follow in the next patches:
FunctionalPort      > PortProxy
TranslatingPort     > SETranslatingProxy
VirtualPort         > FSTranslatingProxy

This patch does not instantiate any of the aforementioned ports, it
merely adds the source files.

util/regress passing all ignoring failing eio and t1000