gnulib: Upgrade Gnulib and switch to bootstrap tool

Upgrade Gnulib files to 20190105.

It's much easier to maintain GRUB's use of portability support files
from Gnulib when the process is automatic and driven by a single
configuration file, rather than by maintainers occasionally running
gnulib-tool and committing the result.  Removing these
automatically-copied files from revision control also removes the
temptation to hack the output in ways that are difficult for future
maintainers to follow.  Gnulib includes a "bootstrap" program which is
designed for this.

The canonical way to bootstrap GRUB from revision control is now
"./bootstrap", but "./autogen.sh" is still useful if you just want to
generate the GRUB-specific parts of the build system.

GRUB now requires Autoconf >= 2.63 and Automake >= 1.11, in line with
Gnulib.

Gnulib source code is now placed in grub-core/lib/gnulib/ (which should
not be edited directly), and GRUB's patches are in
grub-core/lib/gnulib-patches/.  I've added a few notes to the developer
manual on how to maintain this.

Signed-off-by: Colin Watson <cjwatson@ubuntu.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/lib/gnulib-patches/fix-null-deref.patch

@@ -0,0 +1,13 @@
+diff --git a/lib/argp-parse.c b/lib/argp-parse.c
+index 6dec57310..900adad54 100644
+--- a/lib/argp-parse.c
++++ b/lib/argp-parse.c
+@@ -940,7 +940,7 @@ weak_alias (__argp_parse, argp_parse)
+ void *
+ __argp_input (const struct argp *argp, const struct argp_state *state)
+ {
+-  if (state)
++  if (state && state->pstate)
+     {
+       struct group *group;
+       struct parser *parser = state->pstate;