nghttpx: Rewrite logging system

This change rewrites logging system of nghttpx.  Previously access log
and error log are written to stderr or syslog and there was no option
to change stderr to something else.  With this change, file path of
access log and error log can be configured separately and logging to
regular file is now added.  To support rotating log, if SIGUSR1 signal
is received by nghttpx, it closes the current log files and reopen it
with the same name.  The format of access log is changed and has same
look of apache's.  But not all columns are not supported yet.
nghttpx: Use %lld instead of PRId64
nghttpx: Cache time for logging
nghttpx: Do not compile pthread_sigmask if NOTHREADS is defined
nghttpx: Log user-agent as "-" if it is empty
nghttpx: Remove S_IROTH when creating log file
nghttpx: Include pid in errorlog
nghttpx: Use std::atomic for Config::cached_time
nghttpx: Use std::shared_ptr to share cached time

To ensure that cached time buffer is not altered while referencing it,
we rely on inerlocking in std::shared_ptr to acheive this.
nghttpx: Cleanup create_evlistener
nghttpx: Log filename and line number after severity in one line
Update android-config and fix build warning with android NDK
nghttpx: Only use thread_local if NOTHREADS is not defined
Disable threading for android build because it lacks thread local storage
nghttpx: Use 431 instead of 413
src: Add additional HTTP status codes defined in RFC 6585
doc: Generate HTML version manual page from manpage
Send additional debug info when terminating session
Check NGHTTP2_GOAWAY_SEND and NGHTTP2_GOAWAY_RECV flags explicitly
nghttpd: Don't shutdown session if pending data is left