lib/gis: improve diagnostic capabilities for compilers and/or static analyzers

[nilason]

Improve diagnostic capabilities for compilers and/or static analyzers, for G_malloc and friends.

Some (random) references:

• https://best.openssf.org/Compiler-Hardening-Guides/Compiler-Annotations-for-C-and-C++.html
• https://clang.llvm.org/docs/AttributeReference.html#ownership-holds-ownership-returns-ownership-takes-clang-static-analyzer
• https://clang.llvm.org/docs/analyzer/checkers.html#unix-malloc-c
• https://developers.redhat.com/blog/2021/04/30/detecting-memory-management-bugs-with-gcc-11-part-1-understanding-dynamic-allocation

[nilason]

GCC runner fails, as proof this works as expected:

gcc  -std=gnu17 -fPIC -Wall -Wextra -isystem/usr/include/gdal -Wpedantic -Werror  -I/home/runner/work/grass/grass/dist.x86_64-pc-linux-gnu/include -I/home/runner/work/grass/grass/dist.x86_64-pc-linux-gnu/include     -I/usr/include/gdal -I/usr/include -DPACKAGE=\""grasslibs"\"   -I/home/runner/work/grass/grass/dist.x86_64-pc-linux-gnu/include -I/home/runner/work/grass/grass/dist.x86_64-pc-linux-gnu/include -DRELDIR=\"lib/vector/Vlib\" -o OBJ.x86_64-pc-linux-gnu/map.o -c map.c
map.c: In function ‘Vect_rename’:
map.c:352:5: error: ‘free’ called on pointer returned from a mismatched allocation function [-Werror=mismatched-dealloc]
  352 |     free(fields);
      |     ^~~~~~~~~~~~
In file included from /home/runner/work/grass/grass/dist.x86_64-pc-linux-gnu/include/grass/gis.h:734,
                 from /home/runner/work/grass/grass/dist.x86_64-pc-linux-gnu/include/grass/vector.h:3,
                 from map.c:28:
/home/runner/work/grass/grass/dist.x86_64-pc-linux-gnu/include/grass/defs/gis.h:131:25: note: returned from ‘G__malloc’
  131 | #define G_malloc(n)     G__malloc(RELDIR "/" __FILE__, __LINE__, (n))
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
map.c:288:21: note: in expansion of macro ‘G_malloc’
  288 |     fields = (int *)G_malloc(n * sizeof(int));
      |                     ^~~~~~~~
cc1: all warnings being treated as errors

[nilason]

Added returns_nonnull to G_malloc, G_calloc, G_realloc as they always return a non-null value (and call G_fatal_error if failed).