[Feat] Fix more compiler warnings

[nilason]

This is a followup feature request to #1247, which addressed a great number of compiler warnings on both gcc and clang.
Those warnings were the compiler's default warnings.
Unleashing the compiler with -Wall will result in a new wave of warnings, which in many cases are perhaps harmless. But who knows what hidden or potential bugs are waiting there.

(Incomplete) list of warnings:

• -Wtautological-overlap-compare checks: Fix -Wtautological-overlap-compare compiler warning #2157
• -Wunused-variable checks: Fix -Wunused-variable compiler warnings #2158
• -Wunused-const-variable checks: Fix -Wunused-const-variable compiler warnings #2159
• -Wunused-lambda-capture checks: Fix -Wunused-lambda-capture compiler warning #2160
• -Wunused-private-field checks: Fix -Wunused-private-field compiler warning #2161
• -Wself-assign checks: Fix -Wself-assign compiler warnings #2162
• -Wunused-function checks: Fix -Wunused-function compiler warnings #2163
• -Wuninitialized checks: Fix -Wuninitialized compiler warnings #2164
• -Wsometimes-uninitialized checks: Fix -Wsometimes-uninitialized compiler warnings #2165
• -Wunused-result lib/display: issue warning on GRASS_NOTIFY command failure #2705 libdisplay: remove redundant GRASS_NOTIFY #2135 checks: Fix -Wunused-result compiler warnings #2166 (see also [Bug] Warnings for -Wunused-result in GCC build #2128)
• -Wstringop-overflow= lib/shapelib: upgrade from upstream #2694 checks: Fix Wstringop-overflow and -Warray-parameter compiler warnings #2733
• -Wformat-truncation= lib/shapelib: upgrade from upstream #2694
• -Wformat-overflow= checks: Fix '-Wformat-overflow=' compiler warnings #2700
• -Wunused-but-set-variable checks: Fix -Wunused-but-set-variable compiler warnings, part 1 #2648 checks: Fix -Wunused-but-set-variable compiler warnings, part 2 #2650
• -Wunused-variable checks: Fix -Wunused-variable warnings #2651
• -Wparentheses checks: Fix -Wparentheses warning #2652
• -Wmaybe-uninitialized checks: Fix -Wmaybe-uninitialized warnings #2653
• -Wunneeded-internal-declaration checks: Fix -Wunneeded-internal-declaration warning #2654
• -Wclass-memaccess r.terraflow: use default #2660
• -Wdeprecated-declaration r.terraflow: use default #2660
• -Warray-parameter= checks: Fix Wstringop-overflow and -Warray-parameter compiler warnings #2733

[nilason]

The CI runners GCC C/C++ standards check (GCC 9.4) and "macOS / macOS build (Clang 14) both build successfully with -Wall and -Werror, as well as the Ubuntu test runners also builds with -Werror.