1) if buffer size is smaller than arguments size, return buffer
size, not arguments size as before.
2) if buffer size is 0, return 0, not EOF as before.
(now it is compatible with Linux and Apache implementations too).
NOTE: Single Unix specs says:
If the value of n {buffer size} is zero on a call to snprintf(), an
unspecified value less than 1 is returned.
It means we can't return EOF since EOF can take *any* value in general
not especially < 1. Better variant will be return -1 (it is less then
1 and different with n == 1 case) but -1 value is already occuped by
EOF in our implementation, so we can't distinguish true IO error
in that case. So 0 here is only possible case still conforming
to Single Unix specs.
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