Fix bug 6673 - smbpasswd does not work with "unix password sync = yes".

Fix bug 6673 - smbpasswd does not work with "unix password sync = yes".
Revert change from 3.3 -> 3.4 with read_socket_with_timeout changed
from sys_read() to sys_recv(). read_socket_with_timeout() is called
with non-fd's (with a pty in chgpasswd.c and with a disk file in
lib/dbwrap_file.c via read_data()). recv works for the disk file,
but not the pty. Change the name of read_socket_with_timeout() to
read_fd_with_timeout() to make this clear (and add comments).
Jeremy.