allow segment size to be set to < 1GiB

Hi,

I was recently reminded of my previous desire to allow setting the segment
size to less than 1GB. It's pretty painful to test large amount of segments
with a segment size of 1GB, certainly our regression test don't cover anything
with multiple segments.

This likely wouldn't have detected the issue fixed in 0e758ae89a2, but it make
it easier to validate that the fix doesn't break anything badly.

In the attached patch I renamed --with-segsize= to --with-segsize-mb= /
-Dsegsize= to -Dsegsize_mb=, to avoid somebody building with --with-segsize=2
or such suddenly ending up with an incompatible build.

Greetings,

Andres Freund