Fix "source" with interpreter-exec

PR mi/15811 points out that "source"ing a file that uses
interpreter-exec will put gdb in a weird state, where the CLI stops
working.  The bug is that tui_interp::suspend does not unregister the
event file descriptor.

The test case is from Andrew Burgess.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=15811



3 files changed