Add support for bash PROMPT_COMMAND being an array

[ceamac]

Starting with bash 5.1, PROMPT_COMMAND can be an array. Detect this
case and append and entry to the array instead of appending to a string.

The current behaviour is appending the string to the first element in the array.

Testing for bash >= 5 is sufficient, because the @a operator exists in
5.0. We need eval on the "then" branch because bash 1.x cannot even
parse that line (it does not support arrays). Tested with bash 1.14.