Handle sysconf results more carefully

Handle sysconf results more carefully

Sysconf indicates that there's no definite limit by returning -1 and
leaving errno unchanged.  In that case, for SC_ARG_MAX, guess 4096.  For
SC_PAGE_SIZE, die, since various operations currently require a
page_size.

Thanks to Mark J Hewitt for reporting the issue, and to Mark and Nix for
investigating the cause.

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