maint: doc and maintainer-check in preparation for 1.17.90.

maint: doc and maintainer-check in preparation for 1.17.90.

* HACKING: mention thanks to individuals.
* NEWS: update for 1.17.90.
* configure.ac (AC_INIT): now 1.17.90.
* m4/amversion.m4 (AM_AUTOMAKE_VERSION): 1.17.90 (auto-update).
* maintainer/syntax-checks.mk (sc_tests_command_subst): allow
whitespace before # shell comment line.
(sc_tests_plain_sleep): emphasize using $sleep without a number;
only replace sleep 1 or sleep 2; longer than that would be fine.
* t/tap-stderr-prefix.tap: use $sleep instead of sleep 1.