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 7d85383edb. Fixes #16617.

src/shared/mount-util.h

@@ -21,13 +21,11 @@
  * PID1 because 16MB of free space is required. */
 #define TMPFS_LIMITS_RUN             ",size=20%,nr_inodes=800k"
 
-/* The limit used for various tmpfs mounts, but not /tmp itself.
+/* The limit used for various nested tmpfs mounts, in paricular for guests started by systemd-nspawn.
  * 10% of RAM (using 16GB of RAM as a baseline) translates to 400k inodes (assuming 4k each) and 25%
  * translates to 1M inodes.
- * /tmp is configured through a .mount unit file. */
-#define TMPFS_LIMITS_TMP             ",size=10%,nr_inodes=400k"
-#define TMPFS_LIMITS_DEV_SHM         TMPFS_LIMITS_TMP
-#define TMPFS_LIMITS_TEMPORARY_FS    TMPFS_LIMITS_TMP
+ * (On the host, /tmp is configured through a .mount unit file.) */
+#define NESTED_TMPFS_LIMITS          ",size=10%,nr_inodes=400k"
 
 /* More space for volatile root and /var */
 #define TMPFS_LIMITS_VAR             ",size=25%,nr_inodes=1m"