illumos gate

I've put together an implementation of this.

For testing, I created a test user and added the test user to the system. I then ran a simple test program that would invoke getgrouplist:

root@pi:~# ./group test 5
ngroups_max = 16
getgrouplist returned -1 (ngids = 16)
getgrouplist returned 16
test: test(1000), test2(1002), test3(1003), test5(1004), test6(1006), test7(1007), test8(1008), test9(1009), test10(1010), test11(1011), test12(1012), test13(1013), test14(1014), test15(1015), test16(1016), test17(1017)
getgrouplist returned 16 (ngids = 16)
root@pi:~# ./group test 16
ngroups_max = 16
getgrouplist returned 16
test: test(1000), test2(1002), test3(1003), test5(1004), test6(1006), test7(1007), test8(1008), test9(1009), test10(1010), test11(1011), test12(1012), test13(1013), test14(1014), test15(1015), test16(1016), test17(1017)
getgrouplist returned 16 (ngids = 16)

git commit f2c438c5058c64b7373448f239156bf60009abcb

commit  f2c438c5058c64b7373448f239156bf60009abcb
Author: Jason King <jason.king@joyent.com>
Date:   2020-12-09T05:57:56.000Z

    7024 Add getgrouplist() to illumos
    Reviewed by: Robert Mustacchi <rm@fingolfin.org>
    Approved by: Dan McDonald <danmcd@joyent.com>