Fix problem when account names contains regexp characters

'mu4e-multi-get-msg-account' fails when account name contains regular
expression characters such as ".". The reason is that account-list gets
double quoted for regular expressions: once from 'regexp-quote' and
another from 'regexp-opt'.

From the doc of 'regexp-opt':
  Each string should be unique in STRINGS and should *not* contain any
  regexps, quoted or not.