Add tests on syscalls that use two file descriptors.

Test sendfile, tee (Linux), splice (Linux) and also
vmsplice (Linux).
Add some pending checks for accept()-generated FDs.

Should the FD produced by an accept() call on a capability
socket descriptor be a capability itself?
Add Linux-specific tests, starting with timer FD manipulation
Add (Linux-specific) timerfd test
Add (Linux-specific) simple eventfd test
Add (Linux-specific) epoll test
Add (Linux-specific) tests for {i,fa}notify FDs.

Note that the FDs produced by fanotify(7) are not currently
capabilities, even if the original directory FD monitored was
a capability.