d/libpam-modules.preinst: Avoid using pidof

The use of pidof creates an implicit depency on sysvinit-utils. sysvinit-utils is currently Essential, but that may change in the future.

This specific use of pidof (finding if a process is running) is simple enough to be replaced with grep'ing /proc/*/cmdline.

This commit aligns the check performed by libpam-modules to the equivalent check performed by glibc.