procfs-util: fix confusion wrt. quantity limit and maximum value

From packit/rawhide-arm64 logs:
Assertion 'limit >= INT_MAX || get_process_ppid(limit+1, NULL) == -ESRCH' failed at src/test/test-process-util.c:855, function test_get_process_ppid(). Aborting.
――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――

The kernel has a few different limits. In particular kernel.threads-max can be
set to some lower value, and kernel.pid_max can be set to a higher value. This
is nice because it reduces PID reuse, even if the number of threads that is
allowed is limited. But the tests assumed that we cannot have a thread with
PID above MIN(kernel.threads-max, kernel.pid_max-1), which is not valid.

So let's rework the whole thing: let's expose the helpers to read
kernel.threads-max and kernel.pid_max, and print what they return in tests.
procfs_tasks_get_limit() was something that is only used in tests, and wasn't
very well defined, so let's drop it.

Fixes #21193.

src/basic/procfs-util.h

@@ -5,7 +5,9 @@
 
 #include "time-util.h"
 
-int procfs_tasks_get_limit(uint64_t *ret);
+int procfs_get_pid_max(uint64_t *ret);
+int procfs_get_threads_max(uint64_t *ret);
+
 int procfs_tasks_set_limit(uint64_t limit);
 int procfs_tasks_get_current(uint64_t *ret);