libc-wrapper: introduce a tiny libc wrapper

Then, move syscall definitions to the wrapper, and prototypes are moved
to relevant headers.

This also adds checks for add_key() and request_key(), as one day
glibc may be going to add some of them separatedly.

The check for fspick in meson.build is dropped, as it is currently
unused in our code.

This also moves
- basic/missing_bpf.h -> include/override/linux/bpf.h,
- basic/missing_keyctl.h -> include/override/linux/keyctl.h.

src/basic/process-util.c

@@ -34,7 +34,6 @@
 #include "locale-util.h"
 #include "log.h"
 #include "memory-util.h"
-#include "missing_syscall.h"
 #include "mountpoint-util.h"
 #include "namespace-util.h"
 #include "nulstr-util.h"