Error on all C/C++ compiler warnings in src/ and tests/

[aloisklink]

Enables the GCC/Clang -Werror flag on everything in the src/ and tests/ folder.

It's only enabled if -DCMAKE_BUILD_TYPE=Debug. This is so that any 3rd parties using edgesec (for example https://github.com/nqminds/manysecured-openwrt-packages or even the .deb build) should still be able to compile edgesec, even if they have a newer version of GCC/Clang that adds new warnings.

These warnings are usually pretty useful, so we probably want to fix them all. If we ever want to disable this, we can do:

add_compile_options(-Wno-error=foo) # too many -Wfoo warnings

Or better yet, we can use target_compile_options to disable them just in one file:

target_compile_options(my_library_with_warnings
  PUBLIC -Wno-error=foo # error is also disabled for all dependencies of this lib (e.g. error is in header file)
  PRIVATE -Wno-error=foo # error is not disable for dependencies of this lib (e.g. error is only in .c source file
)

There was only one minor warning left to fix in our code, see test(os): fix const correctness argv warning.

(OpenSSL, lib/libnetlink, sqlite3 and libpcap print a lot of warnings, but since they're not in src/ and tests/, they won't be converted to errors.

[codecov]

Codecov Report

Merging #292 (1a868b9) into main (146fc1c) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 1a868b9 differs from pull request most recent head bb72c63. Consider uploading reports for the commit bb72c63 to get more accurate results

@@            Coverage Diff             @@
##             main     #292      +/-   ##
==========================================
+ Coverage   49.25%   49.27%   +0.01%     
==========================================
  Files         116      116              
  Lines       18633    18633              
==========================================
+ Hits         9178     9181       +3     
+ Misses       9455     9452       -3     

Impacted Files	Coverage Δ
tests/utils/test_os.c	98.38% <100.00%> (+0.01%)	⬆️
src/utils/sockctl.c	49.39% <0.00%> (+0.58%)	⬆️

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[mereacre]

Great. We need to document(write as comment) in the main CMakeLists file how to disable teh error for specific targets.