More fixes to debug output
Checks existing tables in db_init()
Checks and propagates return codes from database
Makes -1 the failure return code
Lets SQL allocate process identifiers
Ignores dpkg's 'diversion by <xxx> [from/to]: '

Sometimes dpkg -S will return that *in addition to* the wanted pkgname:
line we are looking for. Just ignore these.
Adds Python 3-compatible compat_execfile()
Improves serialization
Adds trace --continue to travis configuration