build: update gnulib submodule, bootstrap and tests/init.sh to latest

* gnulib: Update to latest.
* bootstrap: Update from gnulib.
* tests/init.sh: Likewise.
maint: prevent update-copyright from updating files from gnulib

The files 'bootstrap' and 'tests/init.sh' are taken from the
gnulib submodule.  Therefore, 'make update-copyright' there should
already have updated the copyright year in these 2 files.

* .x-update-copyright: Add entries for the above mentioned files.
maint: update all copyright year number ranges

Run "make update-copyright", but then also run this,
  perl -pi -e 's/2\d\d\d-//' tests/sample-test
to make that one script use the single most recent year number.