[bug-gnu-libiconv] Libiconv iconv_string(...) cannot ignore incorrect parts

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Dear Sir or Madam <bug-gnu-libiconv@gnu.org>
Hello,

I would like to provide some suggestions regarding my usage of the function 
iconv_string in programming. It has come to my attention that this function is 
unable to handle certain parts of a multi-byte string that contains errors. 
Oftentimes, there may be a segment of garbled characters within a string that 
requires ignore, causing iconv_string to be unable to handle the correct parts 
of the string. 

To address this issue, my suggestion is to modify the function to include an 
ignore errors feature. Specifically, I propose the following changes: 

```
/*
If the boolean parameter "strict" is set to 1, input errors will be handled in 
a strict manner. If an error is encountered, encoding will stop immediately and 
return an error code.
If the boolean parameter "strict" is set to 0, input errors will be handled in 
a relaxed manner. If an error is encountered, it will be skipped over and the 
return value will always be 0.
*/
int iconv_string (const char* tocode, const char* fromcode,
                  const char* start, const char* end,
                  char** resultp, size_t* lengthp,
                  bool strict);
```

To avoid compilation errors for older programs that use iconv_string, I suggest 
make a new function "iconv_string_extend". These changes would not only address 
the issue that I have encountered, but also enhance the compatibility and 
expandability of the program. 
Like this:
```
int iconv_string (const char* tocode, const char* fromcode,
                  const char* start, const char* end,
                  char** resultp, size_t* lengthp);
int iconv_string_extend (const char* tocode, const char* fromcode,
                  const char* start, const char* end,
                  char** resultp, size_t* lengthp,
                  bool strict);
```
That is all, thank you.
I am weak in English. If there are some mistakes in this email, please ignore 
it :)

Yours ChenPi11 <wushengwuxi-msctinoulk@outlook.com>
2023/4/2