Registration for SIGHUP notification - for loggers interested
in file rotation on SIGHUP.
Add a rotation flag to LogFileCtx which loggers can use to register
for log rotation.  Have the LogFileCtx handle the log rotation.
Add macros for access to the underlying buffer and offset.
Useful for using passing the buffer through to another writer
such as LogFileCtx.
Add signal based file rotation for:
- alert debug log
- fast log
- stats log
- dns log
- drop log
- file log
- http log
- tls log
- eve/json log
Unregister for file rotation notification when a context is
de-initialized.  Required for unix-socket mode where
contexts come and go.