Treat char[] as single-value types (Potentially breaking change)

[remkop]

(Related to #648, #1834 and #1838):

Currently, the picocli parser treats options and positional parameters with type char[] as array (multi-value) options.
There is some special logic for interactive char[] options, but this is fragile, as pointed out in #1834.

A better solution is to treat all char[] options as single-value options, similar to String options.

For end users, this means that existing applications that use non-interactive char[] options will no longer allow multiple characters to be specified separately on the command line. That is, input like -c A -c B -c C will be rejected and the user needs to specify -c ABC instead.

Applications that want to preserve the previous behaviour will need to change their code to use java.lang.Character[] instead of char[].