nghttpx: Split monolithic one process into control and worker processes

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.

doc/nghttpx.h2r

@@ -42,6 +42,14 @@ SIGUSR2
   After new process comes up, sending SIGQUIT to the original process
   to perform hot swapping.
 
+.. note::
+
+  nghttpx consists of 2 processes: one process for processing these
+  signals, and another one for processing requests.  The former spawns
+  the latter.  The former is called master process, and the latter is
+  called worker process.  The above signal must be sent to the master
+  process.  If the worker process receives one of them, it is ignored.
+
 SERVER PUSH
 -----------
 

src/Makefile.am

@@ -125,6 +125,8 @@ NGHTTPX_SRCS = \
 	shrpx_memcached_connection.cc shrpx_memcached_connection.h \
 	shrpx_memcached_request.h \
 	shrpx_memcached_result.h \
+	shrpx_worker_process.cc shrpx_worker_process.h \
+	shrpx_process.h \
 	buffer.h memchunk.h template.h
 
 if HAVE_SPDYLAY