net: Add PCP and NATPMP implementation

Add a RFC 6886 NATPMP and RFC 6887 Port Control Protocol (PCP)
implementation, to replace libnatpmp.

Author: laanwj

src/util/CMakeLists.txt

@@ -16,6 +16,7 @@ add_library(bitcoin_util STATIC EXCLUDE_FROM_ALL
   hasher.cpp
   moneystr.cpp
   netif.cpp
+  pcp.cpp
   rbf.cpp
   readwritefile.cpp
   serfloat.cpp