curl: Restore noprogress and isatty config values.

The noprogress and isatty in Configurable are global, in a sense
that they persist in one curl invocation. Currently once one
download writes its response data to tty, they are set to FALSE
and they are not restored on successive downloads.  This change
first backups the current noprogress and isatty, and restores
them when download does not write its data to tty.