The existing (incorrect) overriding mechanism is:

Non-minus-one var value overrides table param overrides default value.

Before MDEV-27169, unspecified var value is -1. So if the user sets
both the var to be a value other than -1 and the table param, the var
value will prevail, which is incorrect.

After MDEV-27169, unspecified var value is default value. So if the
user does not set the var but sets the table param, the default value
will prevail, which is even more incorrect.

This patch fixes it so that table param, if specified, always
overrides var value, and the latter if not specified or set to -1,
falls back to the default value

We achieve this by replacing all such overriding in spd_param.cc with
macros that override in the correct way, and removing all the
"overriding -1" lines involving table params in
spider_set_connect_info_default() except for those table params not
defined as sysvar/thdvar in spd_params.cc

We also introduced macros for non-overriding sysvar and thdvar, so
that the code is cleaner and less error-prone

In server versions where MDEV-27169 has not been applied, we also
backport the patch, that is, replacing -1 default values with real
default values

In server versions where MDEV-28006 has not been applied, we do the
same for udf params