The password-command option does not parse its value correctly.
Quotes are ignored, making many kinds of commands impossible to
express (e.g.  `sh -c "foo | bar"`).  Also, `cabal user-config`
treats the argument list as a *list of option values*, rather than a
*value that is a list*.  As a consequence, `cabal user-config
update` corrupts the value in the config file.

Fix these issues by parsing the command as a space separated list of
tokens, and changing the getter to `unwords` the value and return a
*singleton* list.  Also update the argument placeholder from
`PASSWORD` to `COMMAND`.

Fixes: https://github.com/haskell/cabal/issues/6268