Update build information

Add the command necessary to add ppa repository in Ubuntu

Refs #3757
Re-enable UPnP by default in gitian builds

IIRC this was the case with 0.8.6, so let's keep this to avoid the risk
of losing connectable nodes with 0.9 release.

Also our miniupnpc library was recently updated and I've heard
reports that it works better than before now.
Merge pull request #3762

93c3e21 Re-enable UPnP by default in gitian builds (Wladimir J. van der Laan)
Merge pull request #3758

f8a7b50 Update build information (Drak)
qt: Modernize 'confirmed' terminology in shown tx status

These days we regard transactions with one confirmation to be
'Confirmed'.
Waiting for 6 confirmations is a recommendation but should not
keep the transaction shown as unconfirmed.

Misc code sanity:

- Merge maturity/status enums, they had become completely disjunct
- 'confirmed' flag is now called 'countsForBalance' for clarity
Merge pull request #3719

f642fd9 qt: Modernize 'confirmed' terminology in shown tx status (Wladimir J. van der Laan)