fetchmail: update to 6.4.2

upstream changes:
-----------------
fetchmail-6.4.2 (released 2020-02-14, 27473 LoC):

## BREAKING CHANGES:
* fetchmailconf now supports Python 3 and currently requires the "future"
  package, see https://pypi.org/project/future/.
* fetchmailconf: The minimum supported version is now Python 2.7.13, but it is
  recommended to use at least 2.7.16 (due to its massive SSL updates).
  Older Python versions may check SSL certificates not strictly enough,
  which may cause fetchmail to complain later, if the certificate verify fails.
* fetchmailconf now autoprobes SSL-wrapped connections (ports 993 and 995 for
  IMAP and POP3) as well and by preference.
* fetchmailconf now defaults newly created users to "ssl" if either of the
  existing users sets ssl, or if the server has freshly been probed and
  found supporting ssl.
  There is a caveat: adding a user to an existing server without probing it
  again may skip adding ssl. (This does not prevent STARTTLS.)

## BUG FIXES:
* Fix three bugs in fetchmail.man (one unterminated string to .IP macro, one
  line that ran into a .PP macro, .TH date format), and remove one .br request
  from inside the table, which is unsupported by FreeBSD 12's mandoc(1)
  formatter.  FreeBSD Bug#241032, reported by Helge Oldach.
* Further man page fixes and additions by Chris Mayo and Gregor Zattler.
* When evaluating the need for STARTTLS in non-default configurations (SSL
  certificate validation turned off), fetchmail would only consider --sslproto
  tls1 as requiring STARTTLS, now all non-empty protocol versions do.
* fetchmailconf now properly writes "no sslcertck" if sslcertck is disabled.
* fetchmailconf now catches and reports OS errors (including DNS errors) when
  autoprobing.  Reported as Gitlab issue #12 by Sergey Alirzaev.
* fetchmailconf received a host of other bugfixes, see the Git commit log.

## CHANGES:
* Make t.smoke more robust and use temporary directory as FETCHMAILHOME, to make
  sure that the home directory resolves for the user running the test suite
  even if the environment isn't perfect. Reported by Konstantin Belousov,
  analysed by Corey Halpin, FreeBSD Bug#240914.

## UPDATED TRANSLATION - THANKS TO:
* zh_CN: Boyuan Yang [Chinese (simplified)]


(triaxx)