capability-util: avoid thread_local

capability-util: avoid thread_local

While stracing PID1's forking off of children I noticed that every
single forked off child reads cap_last_cap from procfs. That value is a
kernel constant, hence we can save a lot of work if we'd cache it.

Thing is, we actually do cache it, in a thread_local cache field. This
means that the forked off processes (which are considered new threads)
will have to re-query it, even though we already know the result.

Hence, let's get rid of the thread_local stuff (given that the value is
going to be the same for all threads anyway, and we pretty much have a
single thread only anyway). Use an C11 atomic_int instead, which ensures
the value is either initialized or not initialized, but we don't need to
be concerned of partial initialization.

This makes the cap_last_cap reading go away in the children, as strace
shows (since cap_last_cap() is already called by PID 1 before
fork()ing, anyway).