lint: take care of various warnings/errors issued by cppcheck

- [ ] ~~4860~~ 1688  style (unusedStructMember)
- [x]   ~~68~~ ~~51  error (nullPointer)~~
- [x]   ~~15  style (unreadVariable)~~
- [x]    ~~12  performance (redundantAssignment)~~
- [x]      ~~8  style (unusedVariable)~~
- [ ]      8  performance (passedByValue)
- [x]      ~~7  style (unassignedVariable)~~
- [x]     ~~3  warning (nullPointer)~~
- [x]      ~~1  warning (uselessAssignmentPtrArg)~~
- [x]      ~~1  warning (invalidPrintfArgType_sint)~~
- [x]      ~~1  style (obsoleteFunctionsasctime)~~
- [x]      ~~1  performance (redundantCopy)~~
- [x]      ~~1  error (syntaxError)~~

See #480 

OK, anyhow it appears the preprocessor capabilities are very limited.
```diff
diff --git a/cpu/stm32f4/periph/pwm.c b/cpu/stm32f4/periph/pwm.c
index af68c2c..d8db9cd 100644
--- a/cpu/stm32f4/periph/pwm.c
+++ b/cpu/stm32f4/periph/pwm.c
@@ -25,8 +25,9 @@
 #include "periph/pwm.h"
 #include "periph_conf.h"
 
+#define PWM_DISABLED (!(PWM_0_EN || PWM_1_EN || PWM_2_EN || PWM_3_EN))
 /* ignore file in case no PWM devices are defined */
-#if PWM_NUMOF
+#if !PWM_DISABLED
 
 int pwm_init(pwm_t dev, pwm_mode_t mode, unsigned int frequency, unsigned int resolution)
 {
@@ -239,4 +240,4 @@ void pwm_poweroff(pwm_t dev)
     }
 }
 
-#endif /* PWM_NUMOF */
+#endif /* !PWM_DISABLED */
```
Leads to none of the content being checked ever, removing the `!` leads to the known warnings.