With projects.h not being available to outside users anymore we need to
define PJ_UNITS, PJ_ELLPS, PJ_PRIME_MERIDIANS and PJ_OPERATIONS
elsewhere.

Related pj_get_*_ref() functions have been removed in favour
of their proj_ namespaced counterparts.

char pointers have been changed to const char pointers.

Resolves #983
Resolved #1147

Make char pointers const

Bletch, pj_init also decodes the to_meters field of pj_unit, but only
handles plain numbers or 1/nnn, but not 1./nnn or mmm/nnn.  (So the
original code would have not decoded the us-in entry properly.)

Probably pj_init should be changed to use the factor field instead.

Alternatively pj_init should look for a "/" anywhere in the field and
decode the numerator and denominator as separate doubles.  This would
be needed if ratios are ever to be entered directly by the user (this
is the strategy used by GeographicLib).  And then, of course, the
functionality should be provided by a separate utility function.

Previously, unit conversion using atof(unit_list[i].to_meter) which
gives the wrong answer with, e.g., "1/10".  Now it directly uses
unit_list[i].factor (e.g., 0.1).

Also fix all the conversion factors for the US Surveyor units so that
they are the closest doubles.  E.g., the conversion factors for US
feet are

        factor              rel error
    old 0.304800609601219   6e-16
        12/39.37            1e-16  
    now 1200/3937.0         5e-17

Maybe someone should check the Indian units (but it's possible that
India and Pakistan have different standards).

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.

* Prevent crashes and leaks on allocation failure

Memory allocation can fail. We need to gracefully handle this case and
prevent dereferencing null pointers.

* Make NULL checks consistent within a file

* Properly report allocation errors

* Improve cleanup in pj_gc_reader.c

* Implement pj_strdup and use instead of strdup

The function strdup is not part of ANSI C 89, but a POSIX extension.
Therefore we can not rely on it being available on all platforms.