Also fixes: MDEV-30050 Inconsistent results of DISTINCT with NOPAD

Problem:

Key segments for CHAR columns where compared using strnncollsp()
for engines MyISAM and Aria.

This did not work correct in case if the engine applyied trailing
space compression.

Fix:

Replacing ha_compare_text() calls to new functions:

- ha_compare_char_varying()
- ha_compare_char_fixed()
- ha_compare_word()
- ha_compare_word_prefix()
- ha_compare_word_or_prefix()

The code branch corresponding to comparison of CHAR column keys
(HA_KEYTYPE_TEXT segment type) now uses ha_compare_char_fixed()
which calls strnncollsp_nchars().

This patch does not change the behavior for the rest of the code:
- comparison of VARCHAR/TEXT column keys
  (HA_KEYTYPE_VARTEXT1, HA_KEYTYPE_VARTEXT2 segments types)
- comparison in the fulltext code