funclatency wrong results with nested function calls

To quote the manpage:

nested or recursive functions are not supported properly, and timestamps will
be overwritten, creating dubious output.

This reduces the usefulness of `funclatency`. I ran into this problem the first time using it, was confused by the output, read the `-v` bpf program, realized it would overwrite the timestamp, and came here to file the bug (thankfully checked the manpage before I finished writing the bug report).

Personally I would prefer that `funclatency` either give accurate output for multiple functions, or only accept one function so that its limitations are clear.

Note, this issue does not apply to `funccount`.

Start with funclatency and its manpage, then inspect the verbose BPF program to trace how timestamps are stored for nested or recursive calls. Reproduce the reported multi-function case and define done as either accurate nested-call output or an explicit single-function restriction, with documentation updated accordingly.

Written by the indexing model from the issue text.