More compiler warnings

[katajakasa]

Instead of bothering with coverity/sonarcloud, we should consider enabling more compiler warnings for better quality code. Currently we only use "-Wall", which enables the conservative warnings.

Try out these:

• "-Wshadow" for shadowed variables (we have lots of these)
• "-Wformat=2" for better printf() etc. format checks
• "-Wuninitialized" for checking if something has been left uninitialized
• "-Wfloat-equal" since we shouldn't really be doing equality checks on floats anyways
• "-Wcast-qual" for lost qualifiers, etc. missing const.
• "-Wpointer-arith" this seems to be recommended, as it helps making sure code works on other compilers than gcc too
• "-Wwrite-strings" for string copy mistakes

I haven't tried any of the above yet, and some (all?) might be too zealous. These should be tested one at a time and see how much we would have to rewrite (and if they are worth it).

Any other flags ?