Code cleanup

- remove unneeded variables
- replace ternary operator with array_splice call
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
WIP: Fix install_date_migrate() for pgsql on ADOdb 5.18

The param count was reset by query execution, causing error in the
function due to mismatched bind parameters when calling db_query_bound()
the second time.

@TODO test, report issue

Fixes #
WIP: 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 #
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: stop ignoring build failures for PostgreSQL

[skip ci]