maint: import modern bootstrap

maint: import modern bootstrap

Copy the latest gnulib bootstrap, which runs autoreconf and
generates po/Makevars for us.  Other improvements include some
improved prerequisite tool checking.

This also fixes a bug in the .pot files, regarding the copyright holder.

* bootstrap: Update to version in .gnulib/build-aux.
* bootstrap.conf (MSGID_BUGS_ADDRESS, COPYRIGHT_HOLDER, SKIP_PO)
(gnulib_mk, ACLOCAL, bootstrap_epilogue): Provide overrides.
* autogen.sh (autoreconf): Avoid redundant autoreconf if bootstrap
was run.
* po/Makevars: Delete, now that bootstrap creates it.
* po/.gitignore: Update.