gdb: Fix gdb -tui mode

gdb: Fix gdb -tui mode

The gdbtui (aka gdb -tui) mode has been nonfunctional for a long time
due to the replacement of readline with libedit.  After speaking with
Cristos Zoulas, it was confirmed that libedit can't support gdbtui now
and is not envisioned to support it in the future.

The tui mode is fixed by building the readline library internally and
linking against that instead of libedit.

The README files were updated to reflected additional imported sources
for readline and the removal of libedit-caused local modications to
tui-io.c file.