Subprocess: support capture of stdout/err to same string
Add time_t features to Date class

- convert Date to a time_t
- Date::now returns current time, with same override rules as Date::today
doxygen fixes
Separate Backup addition logic into Conf::addBackup
Make Backup::contents into a single string rather than a vector
Partially introduce exact timestamps for backups
Wrapper class for database access
Conversion to sqlite format