printn renders +Inf and -Inf as [+/- inf]

[p15-git-acc]

Today this behavior in existing arb test code sent me on the wrong track. I hadn't noticed it earlier because I usually use print or printd not printn.

Issue https://github.com/fredrik-johansson/arb/issues/152 mentions this but it also mentions other things that derail the discussion. That's why I'm making this new issue.

#include "arb.h"
int main()
{
    arb_t x;
    arb_init(x);
    arb_pos_inf(x);
    flint_printf("printn: "); arb_printn(x, 50, 0); flint_printf("\n");
    flint_printf("printd: "); arb_printd(x, 50); flint_printf("\n");
    flint_printf("print: "); arb_print(x); flint_printf("\n");
    arb_clear(x);
    flint_cleanup();
    return 0;
}

printn: [+/- inf]
printd: inf +/- 0
print: (+inf) +/- (0)