Follow-up to sys_ipc restructuration

Follow-up to sys_ipc restructuration
Rev 13078 removed a PRE call (for SEMCTL) in the POST(sys_ipc).
This commit adds the correct POST call for SEMCTL in POST(sys_ipc).
(note: some tests are missing in this area, as removing PRE
and adding POST did not cause any test result to change).

This is the last patch to restructure the sys_ipc code.

After this patch, should be able to do the real fix
for 23837 (semctl GETVAL false positive)

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13079 a5019735-40e9-0310-863c-91ae7b9d1cf9