Dramatically reduce System V shared memory consumption.

Except when compiling with EXEC_BACKEND, we'll now allocate only a tiny
amount of System V shared memory (as an interlock to protect the data
directory) and allocate the rest as anonymous shared memory via mmap.
This will hopefully spare most users the hassle of adjusting operating
system parameters before being able to start PostgreSQL with a
reasonable value for shared_buffers.

There are a bunch of documentation updates needed here, and we might
need to adjust some of the HINT messages related to shared memory as
well.  But it's not 100% clear how portable this is, so before we
write the documentation, let's give it a spin on the buildfarm and
see what turns red.