* check the username fits the format expected by adduser;
* don't allow root;
* if the user exists, don't try to create it; usually, this is
harmless, but it fails if the user exists with a UID outside the
normal range for system users.
Fixes#24.
/etc/default/dump1090-mutability is now generated in postinst,
if it doesn't exist, from a template in /usr/share/dump1090-mutability.
Subsequently, the config file is updated in place if debconf-driven
changes are made.
Static HTML/javascript have moved to a "html" subdir to avoid exposing
the template via the webserver.
This is adapted from the FlightAware fork, with some cleanup and
modifications needed to work with the net-cleanup changes.
Inclusion of "verbatim" TSV data read from an AVR-format input
connection is not supported.
