BUILD/MEDIUM: standard: get rid of sprintf()

BUILD/MEDIUM: standard: get rid of sprintf()

OpenBSD complains about the use of sprintf in human_time() :

src/standard.o(.text+0x1c40): In function `human_time':
src/standard.c:2067: warning: sprintf() is often misused, please use snprintf()

We can easily get around this by having a pointer to the end of the string and
using snprintf() instead.