Make various variables read-only (const)

This allows the variables to be moved from .data to .rodata section which
means that more data can be shared by processes and makes sure that nothing
can accidentally overwrite the read-only definitions.  On a x86-64 Linux
system this moves roughly 7 kilobytes of previously writable data to the
read-only data segment in the backend and 4 kilobytes in libpq.