[core] rename UDP proxy to UDP forward (#3266)

Thread defines UDP proxy as a feature to allow secure, two-way
communication of Thread Management Commands between the Commissioner
and any Thread Device in the Thread Network via the Border Agent.

This commit renames the existing UDP proxy feature which forwards UDP
packets back and forth between Host and NCP to UDP forward.

examples/Makefile-posix

@@ -69,7 +69,7 @@ configure_OPTIONS                   = \
     --enable-radio-only               \
     --enable-raw-link-api             \
     --enable-service                  \
-    --enable-udp-proxy                \
+    --enable-udp-forward              \
     --with-examples=posix             \
     $(NULL)
 

examples/common-switches.mk

@@ -123,8 +123,8 @@ ifeq ($(SNTP_CLIENT),1)
 configure_OPTIONS              += --enable-sntp-client
 endif
 
-ifeq ($(UDP_PROXY),1)
-configure_OPTIONS              += --enable-udp-proxy
+ifeq ($(UDP_FORWARD),1)
+configure_OPTIONS              += --enable-udp-forward
 endif
 
 ifeq ($(DEBUG_UART),1)