miscellaneous fixes & doc: i18n, custom format and vcard

* converted HOWTO.translating_abook to UTF-8, added a couple of lines
 about quickly installing and testing new translations and updated URL
 of some po-related softwares.

* fixed a trivial error with custom format strncpy() initialization

* vcard builtin import: removed variables unused since e3aa1d4

* vcard export: set a "PREF" EMAIL field attribute (not [yet]
  used by vcard import though)

* vcard libvformat import:
  - fixed segfault on 64bits arch where a va_list's NULL sentinel value
    must be properly casted [sizeof(int) != sizeof(char *)]
  - now really import multivalued fields (better use of libvformat):
    VFGP_FIND instead of VFGP_GET + vf_get_next_property()
  - use of abook_list instead of strconcat for multivalued fields
    (groups and emails)
  - no more duplicated phone numbers if one or more *PHONE/FAX is
    provided while no HOMEPHONE is available