Fix bugs when printing plurals of numbers that are not

Fix bugs when printing plurals of numbers that are not
unsigned long int values.
* src/system.h (select_plural): New function.
* src/md5sum.c (digest_check): Use select_plural to avoid bug.
* src/uptime.c (print_uptime): Likewise.
* src/dd.c (print_stats): Likewise.  Also, don't use ngettext to
print a floating point number, as reducing to 0 or 1 doesn't work
for some languages.  Instead, just use "s" for seconds since it
doesn't need a plural form.