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.