ps: Add PSS and USS fields

ps: Add PSS and USS fields

The library added smaps_rollup fields in the referenced commit.
This commit exploits the new fields to give pss and uss options.

These options were first proposed back in 2015 by Petr Malat
and, with the library update, they are finally made it into ps.

Why use proportional or unique segment size? It is argued that
these give a better idea of the "real" memory usage of a process.

References:
 commit 12543b6c7690c379abc28e278cd804c05490a8b1
 issue #112
 https://www.freelists.org/post/procps/PSS-and-USS-support-for-ps
 https://lwn.net/Articles/230975/

Signed-off-by: Craig Small <csmall@dropbear.xyz>