Always use snprintf()'s return value, since discarding it is a style
bug at best and using it here gives slightly simpler code and better
error checking. Use snprintf() in putlongdouble() the same as in
putfloat(). (1.25 changed most sprintf()'s to snprintf()'s to fix
non-bugs without changing the logic to use the result of snprintf();
1.27 restored one of the sprintf()s by cloning a stale version of
putfloat().)
Don't print a too-long field in the unlikely case that the fallback
to M units in putint() leaves the field still too long. (The fallback
to printing stars was lost in rev.1.58 when the fallback to M units
was added.)
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