logging: add option to log to separate file, useful for debug.

This can be simpler than using log to syslog OR log to console for
reporting problems.
access: add new option to override all ACL

This can be quite useful for debug/testing, especially when using
configuration provided by a user (saves deleting ACL files).
dvb: add a bit more debug to NIT processing
Fix #1596 - some version of git don't like describe in subdir
Fix #1595 - fix typo in IPV6 check for debian init script
Fix #1591 - timeshift: fix mutex deadlock affecting on-demand mode.

Note: there is still an issue I've realised in that writer_flush() could
potentially result in a single packet being written out of order as two
threads have the potential to write. This isn't fatal and is probably
rare enough to ignore at this stage.