to support charset change in SWAT

[TAKAHASHI Motonobu]

Andrew, 

I am looking at the SWAT code to support changing unix charset via
SWAT.

it needs:

- to force to call iconv_init() when changing unix charset.
  adding a parameter LIKE "BOOLEAN refresh" to convert_string() 
  if refresh is True, always call init_iconv() regardless of
  initialised.
  It's easy to implement.

- to create a new function similar with convert_string()
  but instead of "charset_t from,to", "char* from, to" to input
  charset name such as CP932 or EUC-JP directly is needed.

  because to change unix charset, write_config()  should call like

    convert_string(before_unix_charset, after_unix_charset, ...);

  but before_unix_charset is not same as any of CH_*.

  make sense?

-----
TAKAHASHI, Motonobu(monyo)         monyo at samba.gr.jp
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