MINOR: cli: support filtering on FD types in "show fd"

MINOR: cli: support filtering on FD types in "show fd"

Depending on what we're debugging, some FDs can represent pollution in
the "show fd" output. Here we add a set of filters allowing to pick (or
exclude) any combination of listener, frontend conn, backend conn, pipes,
etc. "show fd l" will only list listening connections for example.