Add man page generation script using help2man

doc/nghttp.h2m

+[SEE ALSO]
+
+nghttpd(1), nghttpx(1)

doc/nghttpd.h2m

+[SEE ALSO]
+
+nghttp(1), nghttpx(1)

doc/nghttpx.h2m

+[SEE ALSO]
+
+nghttp(1), nghttpd(1)

makemanpages

+#!/bin/sh -e
+
+help2man --output=doc/nghttp.1 --name="HTTP/2 experimental client" \
+    -N --include doc/nghttp.h2m src/nghttp
+
+help2man --output=doc/nghttpd.1 --name="HTTP/2 experimental server" \
+    -N --include doc/nghttpd.h2m src/nghttpd
+
+help2man --output=doc/nghttpx.1 --name="HTTP/2 experimental proxy" \
+    -N --include doc/nghttpx.h2m src/nghttpx