Install: special processing for pgsql boolean columns

The upgrade of ADOdb library to version 4.96 with Mantis release
1.1.0 introduced a change in the way type 'L' columns are created
(SMALLINT before, BOOLEAN after).

This requires special handling to fix the database schema, because the
normal schema upgrade steps do not cover it (since from the MantisBT
installer's perspective the column type has not changed).

Fixes #16392
Install: pgsql bool check now handles nullable columns

Issue #16392
Fix travis build errors for PostgreSQL

The errors were caused by executing queries with params in wrong order.

To fix the problem, this commit adds Database API functions to push the
current parameter number onto a stack and pop the value after query
execution, allowing build of multiple queries concurrently.

The caller can call db_param_push() when needed prior to starting the
build of a new query while another one is still pending execution.
db_query_bound() will call db_param_pop() after running the query.

Fixes #14398
Move parameter count management to a class

The MantisDbParam class handles the logic for:
- assignment of parameters, db_param() calls
- push onto stack, db_param_push() calls
- pop stack
Update ADOdb to latest 5.19dev
Travis before script: use variable for port number

This facilitates local debugging using the PHP built-in web server when
there is another one running on default port 80.
Travis: stop ignoring build failures for PostgreSQL
Coding guidelines

- $v_ prefix for extracted variables
- $t_ prefix for other local variables
Merge PostgreSQL fixes branch

This covers several fixes to (re-)enable support for pgsql database in
master branch, which has been broken since upgrade to unpatched ADOdb
5.11 several years ago. With these, PostgreSQL should now be
operational:

- Correct handling for booleans, including fix of installer/upgrade
  process to convert pre 1.1.0 instances SMALLINT columns to BOOLEAN
- Finally fixing the broken Travis builds. This required new DB API
  calls to push/pop the parameter count to a stack, to enable concurrent
  build of multiple queries (required as pgsql uses positional
  parameters). Initial implementation 124f79e was procedural, then I moved
  this to a simple class in d09d5ad
- ADOdb update to latest development head (includes a required fix to
  ADODB_postgres64->Param() )

Also includes .travis.yml changes to stop ignoring pgsql build failures.

Pull request #143