waitpid: support 'PID:inode' process addressing format

waitpid: support 'PID:inode' process addressing format

Adding support for the 'PID:inode' format to address a
process ensures that waitpid(1) safely polls the status
of the expected process in a race-free manner. Worthy of
note is that with the addition of the getino(1) command
the aforementioned task becomes trivially simple.

Example:
        waitpid $(getino -p 123456)

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