kill: add support for race-free process kills using pidfd inodes

kill: add support for race-free process kills using pidfd inodes

The 6.9 Linux kernel added support for pidfds which introduces inodes
that stay unique for the system lifetime and can be used to reference
a process with both its traditional pid and pid fd inode number.
This enables race-free killing of processes and protects from false
referencing due to pid rollovers.

This patch adds a new way of addressing processes with the format
'pid:pidfd_inode'. When the new format is used, 'kill' assumes the user wishes
to use a pidfd to signal a process, and will therefore use pidfd_* routines
to complete the task if the provided inode matches the one from a previously
acquired pidfd.

Addresses: #3252
Signed-off-by: Christian Goeschel Ndjomouo <cgoesc2@wgu.edu>