Systemd assumes that service of type simple is stopped when the main
process exits. This causes systemd to kill all nghttpx processes when
doing a process upgrade (via USR2/QUIT signals). Change the service
type to forking which behaves correctly on upgrade.

This is meant to be used for ints, but unfortunately std::forward is
not constexpr in C++11.

Instead of assuming that upgrade is always successful, check if a new
process actually started. Do not kill old process if the new one
failed.

Merge branch 'unix-tcp-nodelay' of https://github.com/rraptorr/nghttp2 into rraptorr-unix-tcp-nodelay

This silences warning log that otherwise spams logs on every accepted
connection.