lsfd: Refactor the pidfd logic into lsfd-pidfd.c

lsfd: Refactor the pidfd logic into lsfd-pidfd.c

We'll reuse these logic for pidfd support on Linux >= 6.9.  This should
be a no-functional change.

Besides moving the code, this change also renames anon_pidfd_data to
pidfd_data, and removes a redundant nullity check for free (because
free(NULL) will just do nothing per the C standard).

Signed-off-by: Xi Ruoyao <xry111@xry111.site>