maint: Update after gnulib module 'readutmp' changed

maint: Update after gnulib module 'readutmp' changed

(This patch is coauthored with Bruno Haible,
with original version at <https://bugs.gnu.org/64937#>.)
This updates the gnulib submodule to latest.
For year-2038 safety on Linux/{x86,arm},
this adds an --enable-systemd option to ‘configure’.
The idea is that this sort of thing will become the default
after it has been tested more.
* configure.ac: Don't test whether struct utmp and struct utmpx
have the ut_host field; this is now done in gnulib's readutmp module.
* src/local.mk: Link the programs 'pinky', 'uptime', 'users',
'who' with $(READUTMP_LIB).
* src/pinky.c, src/who.c:
Test HAVE_STRUCT_XTMP_UT_HOST instead of HAVE_UT_HOST.
* src/pinky.c (print_entry):
* src/who.c (print_user, print_deadprocs, print_login)
(print_initspawn, scan_entries):
Support the situation where ut_line is a 'char *' rather than a
'char[]' of fixed size.  Likewise for ut_user and ut_host.
(make_id_equals_comment): Likewise for ut_id.
* src/pinky.c (print_entry):
* src/who.c (print_user):
Open /dev to simplify looking up its entries.
Don’t use printf if the output might in theory be longer than INT_MAX.
* src/pinky.c (scan_entries, short_pinky):
* src/uptime.c (print_uptime, uptime):
* src/users.c (list_entries_users, users):
* src/who.c (who):
Use idx_t where new read_utmp needs it.
* src/system.h (STREQ_LEN): Add comment that last arg can be -1.