library: suppress zero 'days' under the <uptime.h> API

library: suppress zero 'days' under the <uptime.h> API

The former whattime logic used to suppress that 'days'
output when the system had been up less than 24 hours.

But since the refactor for newlib, '0 days' was always
output under those conditions. So this commit restores
the former expected behavior of suppressing that item.

[ plus an erroneous calculation of uphours was fixed ]

[ and the clang warnings shown below were also fixed ]

Reference(s):
proc/uptime.c:74:10: warning: unused variable 'buf' [-Wunused-variable]
    char buf[256];
         ^
proc/uptime.c:131:58: warning: data argument not used by format string [-Wformat-extra-args]
        pos += sprintf(upbuf + pos, "%d min, ", uphours, upminutes);
                                    ~~~~~~~~~~           ^
proc/uptime.c:175:15: warning: expression result unused [-Wunused-value]
        comma +1;
        ~~~~~ ^~

Signed-off-by: Jim Warner <james.warner@comcast.net>