Update HACKING a bit for C99 standard
If we write C99, we better make sure we compile as such
Add info on how g_spawn is broken on Windows
Plug a possible memory leak

There is no need to check doc->real_path since the usage is NULL-safe,
and if it actually is empty or NULL it isn't a bug in the caller itself
anyway, and this could leak.
Simplify utils_get_hex_from_color() and plug a possible memory leak in it
Plug a possible memory leak
Fix mismatching allocation and deallocation

When allocation with g_malloc(), the memory should be freed using
g_free(), not plain free().

Also, use g_try_malloc() instead of g_malloc() where the code carefully
handles allocation failures itself.
Remove an unused variable that could be leaked
Plug a memory leak
Plug a possible memory leak
Explicit a valid cast
Plug a memory leak on Windows
Remove an unused variable and plug a theoretical memory leak
R has braces
Fix some small type issues in printf formats
Fix 2 unlikely memory leaks
Fasten utils_tidy_path() a little and fix possible leaks in it

Fix possible leaks with bad paths, and also speed up some part of the
function by avoiding unnecessary allocations and string length
computations.
Always use proper functions argument lists
Don't depend on an implementation detail of S_ISREG and S_ISDIR
Use proper argument list