Drop pwd/gid reentrant function variants for now

Drop pwd/gid reentrant function variants for now

From the POSIX pages, it sounded like _SC_GETPW_R_SIZE_MAX, etc. were
upper limits, but other docs, and observed behavior (reported ERANGE
failures) indicate that it's more likely they're just suggestions.  As
a result, we'd either need larger hard-coded buffers, or a
reallocate/retry loop if we kept them.

Instead, since we're not at all thread-safe and aren't likely to be
anytime soon, the dynamic allocations are just less efficient and more
complex, so back off to the non-reentrant flavors for now.

Thanks to kd7spq for reporting the problem and Johannes Berg for
tracking down the cause and help settling on this solution.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>