Remove requirement for SZERO=YES

When VSAM was first introduced, the only way, without requiring changes to every ATTACH in the address space, to share user storage was to use subpool zero. Sharing subpool zero has unintended consequences for a task that (ABENDed) ended without freeing their subpool zero storage. Since server address spaces now can run for months and tasks can come and go, subpool zero grows in size. IBM has recognized and has since added subpools 128-131 (and later subpools 132-133 but for a different reason) to allow programs to specifically share storage. Server address spaces can now use SZERO=NO on all attach requests and storage that is not intended to outlast the termination of the TCB will be automatically freed.

VSAM requires SZERO=YES on all TCBs. I was under the impression that if the TCB that opened the ACB did all of the operations, SZERO=YES would not be required since only one TCB was involved. This didn't take into account abnormal situations where VSAM is unable to close the ACB. In this case, the PLH control blocks in subpool zero storage are automatically release (because SZERO=NO) but the address of the PLH chain is saved in the AMB, which is allocated in a different subpool whose ownership is assigned to the job step TCB. When the same file is opened later, VSAM fails S0C4 trying to access the first PLH.

VSAM should use one of the subpools created for explicitly for sharing instead of subpool zero.