chpst: fix a bug where -U USER was using wrong USER (one from -u USER)

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
fix assorted unused code and wrong format specs found by cppchekc (bug 6716)

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Use unsigned printf/scanf conversion where more appropriate

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>