MINOR: debug: Report the current tgid in "debug dev fd"

MINOR: debug: Report the current tgid in "debug dev fd"

"debug dev fd" scans the FDs of the process as the calling thread sees
them, and reports the ones unknown to fdtab. With per-thread-group FD
tables, both that scan and the fdtab lookups only reflect the view of
the calling thread's group: the same command may report different FDs
depending on the thread that serves it. Let's print the current tgid in
a header line so the output states which view it shows.