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
362a55fc14 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
7d85383edbab7. Fixes #16617.