Consolidate all Netatalk specific conversion in pull_charset_flags()

Netatalk specific conversion like '/' <-> ':' and CAP de-/encoding
was performed partially in pull_charset_flags() and push_charset_flags().
The resulting code was unmaintainable and as a result it failed to
to the proper conversion of ':' to '/' for an UTF8 -> UTF8-MAC
conversion.

Moving all Netatalk specific conversion to pull_charset_flags()
we can now easily do the necessary conversion in one place.

* ":xx" CAP sequences are decoded if CONV_UNESCAPEHEX is used
* ':' and '/' are CAP encoded if CONV_ESCAPEHEX is used
* ':' is converted to '/' only for a conversion whos target charset
  is UTF8-MAC and whos source charset is NOT UTF8-MAC
* '/' is converted to ':' only for a conversion whos source charset
  is UTF8-MAC and whos target charset is NOT UTF8-MAC
* a leading '.' is CAP encoded if CONV_ESCAPEDOTS is used

This consolidation obsoletes both CONV_ALLOW_COLON and CONV_ALLOW_SLASH
conversion options.