Support for MSVC compiler: Ensure ssize_t gets defined.

Support for MSVC compiler: Ensure ssize_t gets defined.

* doc/posix-headers/sys_types.texi: Mention the missing ssize_t problem.
* doc/posix-headers/stdio.texi: Likewise.
* modules/stdio (Depends-on): Add ssize_t.
* modules/sys_socket (Depends-on): Likewise.
* modules/sys_types (Depends-on): Likewise.
* modules/sys_uio (Depends-on): Likewise.
* modules/unistd (Depends-on): Likewise.
* tests/test-sys_socket.c: Check that size_t and ssize_t are defined.
* tests/test-sys_types.c: Check that ssize_t is defined.