Makefile.am: test-bus-memfd went away. Kill its residues in Makefile.am
memfd: move code from public library to src/shared

Don't expose generic kernel API via libsystemd, but keep the code internal
for our own usage.
memfd: fix memfd_create() syscall wrapper

Unlike earlier versions, the syscall only takes 2 arguments in its
final version, not 3.