bpftool: Add bash completion for program signing options

bpftool: Add bash completion for program signing options

Commit 40863f4d6ef2 ("bpftool: Add support for signing BPF programs")
added new options for "bpftool prog load" and "bpftool gen skeleton".
This commit brings the relevant update to the bash completion file.

We rework slightly the processing of options to make completion more
resilient for options that take an argument.

Signed-off-by: Quentin Monnet <qmo@kernel.org>
Link: https://lore.kernel.org/r/20250923103802.57695-1-qmo@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>