pidof: support for omitted %PPID and additional separators

This commit introduces support for special %PPID value that
can be passed to the -o option as a substitution for parent
PID. It also allows users to use two additional separators
for omitted PIDs - colon and semicolon.
ps: possibility to display slice unit for a process

Library systemd-login offers possibility to display
name of a systemd slice unit for specific pid.

This patch adds output option "slice" which will
show name of systemd slice unit.

To maintain compatibility with non-systemd systems,
procps must be configured with --with-systemd option
to enable this option.
Merge branch 'master' of git://gitorious.org/procps/procps