* COPYING: Update from current contents of
https://www.gnu.org/licenses/gpl-3.0.txt (no substantive licensing
changes, only updated URLs).
* Makefile.am (ACLOCAL_AMFLAGS): Update URL in comment to use HTTPS.
* README.md (Installation): Update URLs for pkg-config and check.
(Note on GPL versions): Update GNU licence compatibility URL to use
HTTPS.
* lib/pipeline.c (clearenv): Update dovecot reference; the old URL used
HTTP and had gone stale.