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