We want to flag that proj_api_h is now deprecated. With this commit
it is now mandatory to #define ACCEPT_USE_OF_DEPRECATED_PROJ_API_H
before proj_api.h can be included.

proj_api.h is used internally a bunch of places. Therefore
ACCEPT_USE_OF_DEPRECATED_PROJ_API_H has been defined in projects.h
and a few other necessary files to ensure that PROJ compiles.

Closes #836

Having non-const variables of static lifetime or even global scope is
usually a bad idea. These variables are inherently constants, and this
should be enforced.

This required marking some functions as not modifying input parameters
and marking some pointers as pointers to const.

One advantage is that the compiler usually puts const static variables
in a read-only code segment, so they can't be modified physically.
This can be verified with `nm` (on POSIX systems).

To avoid changes to the public API, functions returning non-const
pointers to data tables were left intact, but the returned data may not
be modified. Internally we prefer using the proj_list_* functions over
the pj_get_*_ref functions, because the former return const pointers.

The multistresstest code has been made C89 compliant in the process.

Elim some leaks by initializing PJ.destructor in PJ_ob_tran.c properly
Avoid tests bombing when built with address sanitizer: Repair memory leak in test228.c
Avoid tests bombing when built with address sanitizer: Repair memory leak in multistresstest.c