kcmp: output fds using a separate function

kcmp: output fds using a separate function

This is a preparation for the future introduction of cross-NS PID
derivation, which would enable us to print fd information for fds
related to all processes, not just traced ones.

Note the change in output type for idx1/idx2 in KCMP_FILE command from
unsigned to int, it follows printfd output format.

* kcmp.c (printpidfd): New function.
(PRINT_FIELD_PIDFD): New macro.
(SYS_FUNC(kcmp)) <case KCMP_FILE>: Use printpidfd for printing
idx1/idx2, as they are fds, after all.