trim() functions are lack of utf-8 support

The following string functions are lack of UTF-8 support:

BTRIM(STRING a, STRING chars_to_trim)
LTRIM(STRING a, STRING chars_to_trim)
RTRIM(STRING a , STRING chars_to_trim)

Here is an issue reported by our user:

[localhost:21050] default> select rtrim('价格，', '，');
+-----------------------+
| rtrim('价格，', '，') |
+-----------------------+
| 价�                   |
+-----------------------+

The result is the same if setting utf8_mode=true. Note that the comma used in the above strings is Chinese punctuation mark '，' , not English(ASCII) mark ','.

The cause is that the Chinese character '，' is used as a char set. The utf8 encoding of these characters:

• '价': 0xe4 0xbb 0xb7
• '格': 0xe6 0xa0 0xbc
• '，': 0xef 0xbc 0x8c

Each character is encoded into 3 bytes. The last byte of '格' is 0xbc which also appears in the bytes of '，'. So it's removed as well. The result is a string of '价' and the first two bytes of '格'. The last character becomes a malformed unicode so it's replaced with '�'.