[Libguestfs] server: Fix reading passwords interactively.

https://bugzilla.redhat.com/show_bug.cgi?id=1842440 Patches 1 and 2 address fairly obvious bugs in how we handle reading passwords from stdin. There are other ways we may consider fixing these bugs: - Should password=- always open /dev/tty and ignore stdin entirely? - Should we make password=-0/-1/-2 work by skipping the close? Or perhaps reopen the file descriptors on /dev/null after reading the password? (This seems over-complicated to me, I think -0/-1/-2 is always likely to be a user error). Rich.