free: instead of Curl_safefree()

Since we just started make use of free(NULL) in order to simplify code,
this change takes it a step further and:

- converts lots of Curl_safefree() calls to good old free()
- makes Curl_safefree() not check the pointer before free()

The (new) rule of thumb is: if you really want a function call that
frees a pointer and then assigns it to NULL, then use Curl_safefree().
But we will prefer just using free() from now on.

tests/data/test96

@@ -37,7 +37,7 @@ MEM tool_cfgable.c
 MEM tool_cfgable.c
 </file>
 <stripfile>
-$_ = '' if (($_ !~ /tool_paramhlp/) && ($_ !~ /tool_cfgable/))
+$_ = '' if ((($_ !~ /tool_paramhlp/) && ($_ !~ /tool_cfgable/)) || ($_ =~ /free\(\(nil\)\)/))
 s/:\d+.*//
 s:^(MEM )(.*/)(.*):$1$3:
 </stripfile>