tests: fix build and run on alpha with fresh kernel headers

tests: fix build and run on alpha with fresh kernel headers

Fix tests on alpha with Linux kernel headers containing commits
v5.1-rc1~160^2~3^2~12 and v5.1-rc1~160^2~3^2~1.

* tests/getegid.c: Include "scno.h".
(main): Use sprintrc.
* tests/geteuid.c: Likewise.
* tests/getppid.c: Likewise.
* tests/getgid.c [__NR_getgid == __NR_getxgid]: Skip.
* tests/getuid.c [__NR_getuid == __NR_getxuid]: Skip.
* tests/getpid.c [__NR_getpid == __NR_getxpid]: Skip.
* tests/umount2.c (__NR_umount2): Remove.
(TEST_SYSCALL_NR): New macro.
(main): Use it instead of __NR_umount2.
* tests/ksysent.sed: Remove new __NR_get[gup]id aliases to traditional
__NR_getx[gup]id names (on alpha).
Prioritize __NR_umount over __NR_umount2, __NR_osf_shmat over __NR_shmat.
* NEWS: Mention these fixes.