fetchmail-6.4.3 (WIP)

## BUGFIXES:
* Plug memory leaks when parts of the configuration (defaults, rcfile, command 
  line) override one another.
* fetchmail terminated the placeholder command string too late and included
  garbage from the heap at the end of the string. Workaround: don't use place-
  holders %h or %p in the --plugin string. Bug added in 6.4.0 when merging
  Gitlab merge request !5 in order to fix an input buffer overrun.
  Faulty commit 418cda65f752e367fa663fd13884a45fcbc39ddd.
  Reported by Stefan Thurner, Gitlab issue #16.
* Fetchmail now checks for errors when trying to read the .idfile,
  Gitlab issue #3.
* Fetchmail's error messages that reports that the defaults entry isn't the 
  first was made more precise. It could be misleading if there was a poll or
  skip statement before the defaults.

## CHANGES:
* Fetchmail documentation was updated to require OpenSSL 1.1.1.
  OpenSSL 1.0.2 reached End Of Life status at the end of the year 2019.
  Fetchmail will tolerate, but warn about, 1.0.2 for now on the assumption that 
  distributors backport security fixes as the need arises.
  Fetchmail will also warn if another SSL library that is API-compatible
  with OpenSSL lacks TLS v1.3 support.
* If the trust anchor is missing, fetchmail refers the user to README.SSL.

## INTERNAL CHANGES:
* The AC_DECLS(getenv) check was removed, its only user was broken and not 
  accounting for that AC_DECLS always defines HAVE_DECL_... to 0 or 1, so
  fetchmail never declared a missing getenv() symbol (it was testing with 
  #ifdef).  Remove the backup declaration. getenv is mandated by SUSv2 anyways.

