bpftool: Rename "bpftool feature list" into "... feature list_builtins"

To make it more explicit that the features listed with "bpftool feature
list" are known to bpftool, but not necessary available on the system
(as opposed to the probed features), rename the "feature list" command
into "feature list_builtins".

Note that "bpftool feature list" still works as before given that we
recognise arguments from their prefixes; but the real name of the
subcommand, in particular as displayed in the man page or the
interactive help, will now include "_builtins".

Since we update the bash completion accordingly, let's also take this
chance to redirect error output to /dev/null in the completion script,
to avoid displaying unexpected error messages when users attempt to
tab-complete.

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220701093805.16920-1-quentin@isovalent.com