Cleanup in check_table_name() and check_db_name()

The code in check_table_name() and check_db_name() looks confusing.
Let's do minor cleanups:

• Rename the parameter check_for_path_char to a more self-descriptive disallow_path_chars
• Rename the variable name_length in check_table_name() to a more self-descriptive char_length.
• Fix the while loop in check_table_name() into a for loop and replace multiple char_length+ (the former name_length+) statements to a signle increment statement in the for loop header.