Provide an openpt function for better portability.

Rev: lib/modules/Stdio.pmod/module.pmod:1.173
Rev: src/modules/files/acconfig.h:1.21
Rev: src/modules/files/configure.in:1.88
Rev: src/modules/files/file.c:1.278
Rev: src/modules/files/file_functions.h:1.27
Rev: src/modules/files/testsuite.in:1.30