Base: Add man page for posix_openpt(3)

Base/usr/share/man/man3/posix_openpt.md

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+## Name
+
+posix\_openpt - open a pseudo-terminal device
+
+## Synopsis
+
+```**c++
+#include <stdlib.h>
+#include <fcntl.h>
+
+int posix_openpt(int flags);
+```
+
+## Description
+
+Open a pseudo-terminal master using the given *flags*.
+
+The *flags* argument accepts a bitmask of the following flags:
+
+* `O_RDWR`: Open for both reading and writing.
+* `O_NOCTTY`: The opened pseudo-terminal will not be made the controlling TTY for the process.
+* `O_CLOEXEC`: The opened fd shall be closed on [`exec`(2)](../man2/exec.md).
+
+## Return value
+
+On success, a pseudo-terminal device is allocated and `posix_openpt()` returns a file descriptor for it. Otherwise, it returns -1 and sets `errno` to describe the error.
+
+## Errors
+
+Returns the same errors as [`open`(2)](../man2/open.md).
+
+## See also
+
+* [`open`(2)](../man2/open.md)