pgsql: Add alignment option to ShmemRequestStruct()

Add alignment option to ShmemRequestStruct()

The buffer blocks, converted to use ShmemRequestStruct() in the next
commit, are IO-aligned. This might come handy in other places too, so
make it an explicit feature of ShmemRequestStruct().

Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Matthias van de Meent <boekewurm+postgres@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://www.postgresql.org/message-id/CAExHW5vM1bneLYfg0wGeAa=52UiJ3z4vKd3AJ72X8Fw6k3KKrg@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/dacfe81a0de513308e6753fe3d9b4f3e4811da69

Modified Files
--------------
src/backend/storage/ipc/shmem.c | 39 ++++++++++++++++++++++++++++-----------
src/include/storage/shmem.h     |  6 ++++++
2 files changed, 34 insertions(+), 11 deletions(-)