Minor warnings fixes

Some systems insist on -Wmissing-field-initializers.  We have too many,
by design.  Fix a few easy ones.
Reject tags longer than 4 chars in hb_feature_from_string()
Do not require the '=' in hb_feature_from_string()

Towards accepting CSS font-feature-settings strings.
Fix parsing of features like "- liga" (with the space)
Remove duplicate definition of ISALNUM

It's defined in hb-private.h already.
Allow on/off in hb_feature_from_string()

We now allow things like "dlig on" / "dlig=on".
Minor
Allow space at the end of feature string with values, eg 'dlig=1 '
Allow quotation marks around feature tag in hb_feature_from_string()

With this, I believe we accept CSS feature strings completely.
Make sure broken feature strings are not partially parsed

If user doesn't check hb_feature_from_string() return value, we
don't want them to end up see the partially-parsed feature.
Fix snprintf() format

Patch from Maks Naumov.
Fixes https://github.com/behdad/harfbuzz/pull/22
[util] Note CSS compatibility in --help-features
Typo