This establishes what the "bad" whitespaces are for this
project.
The rules are:
- Unless otherwise specified, indent with SP that could be
replaced with HT are not "bad". But SP before HT in the
indent is "bad", and trailing whitespaces are "bad".
- For C source files, initial indent by SP that can be replaced
with HT is also "bad".
- Test scripts in t/ and test vectors in its subdirectories can
contain anything, so we make it unrestricted for now.
Anything "bad" will be shown in WHITESPACE error indicator in
diff output, and "apply --whitespace=warn" will warn about it.