pid1: stop limiting size of /dev/shm

The explicit limit is dropped, which means that we return to the kernel default
of 50% of RAM. See 362a55fc for a discussion why that is not as much as it
seems. It turns out various applications need more space in /dev/shm and we
would break them by imposing a low limit.

While at it, rename the define and use a single macro for various tmpfs mounts.
We don't really care what the purpose of the given tmpfs is, so it seems
reasonable to use a single macro.

This effectively reverts part of 7d85383e. Fixes #16617.