Fix pinentry for Windows

* pinentry/pinentry.c (pinentry_loop2): Use assuan_fdopen for pipe fds.
* pinentry/pinentry.h (pinentry_loop2): Mention this in the comment.

For Windows calling assuan_fdopen is neccessary as this does
some internal platform specific stuff (get_osfilehandle).

This issue was introduced by using the real libassuan instead
of the built in variant.