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.

The control process handles signals, reads configuration, reads
private keys, and bind port (which may be privileged one).  It never
drop privileges, so that it can execute new binary with the same
privilege.  It forks worker process.  The worker process handles all
incoming connections.  It drops privilege.