Move readline to the readline/readline subdirectory

readline turns out to be a bit of a stumbling block for the project to
move gdbsupport (and then gdbserver) to the top-level.

The issue is that readline headers are intended to be included with
names like "readline/readline.h".  To support this, gdb effectively
adds a -I option pointing to the top-level source directory -- but,
importantly, this option is not used when the system readline is used.

For gdbsupport, a -I option like this would always be needed, but that
in turn would break the system readline case.  This was PR build/17077,
fixed in commit a8a5dbcab8df0b3a9e04745d4fe8d64740acb323.

Previously, we had discussed this on the gdb-patches list in terms of
removing readline from the tree

    https://sourceware.org/ml/gdb-patches/2019-09/msg00317.html

However, Eli expressed some concerns, and Joel did as well (off-list).

Given those concerns, and the fact that a patch-free local readline is
relatively new in gdb (it was locally patched for years), I changed my
mind and decided to handle this situation by moving the readline
sources down a level.

That is, upstream readline is now in readline/readline, and the
top-level readline directory just contains the minimal configury
needed to build that.

This fixes the problem because, when gdb unconditionally adds a
-I$(top_srcdir), this will not find readline headers.  A separate -I
will be needed instead, which is exactly what's needed for
--with-system-readline.

gdb/ChangeLog
2019-10-23  Tom Tromey  <tom@tromey.com>

	* Makefile.in (READLINE_DIR): Update.

gdb/doc/ChangeLog
2019-10-23  Tom Tromey  <tom@tromey.com>

	* Makefile.in (READLINE_DIR): Update.

readline/ChangeLog
2019-10-23  Tom Tromey  <tom@tromey.com>

	Move old contents to readline/ subdirectory.
	* aclocal.m4, configure, configure.ac, .gitignore, Makefile.am,
	Makefile.in, README: New files.

Change-Id: Ice156a2ee09ea68722b48f64d97146d7428ea9e4

readline/.gitignore

@@ -1,37 +1,4 @@
-Makefile
-
-*.o
-*.a
-*.so
-*.sl
-*.dll
-
+autom4te.cache
 config.cache
-config.h
 config.log
 config.status
-
-doc/Makefile
-examples/Makefile
-shlib/Makefile
-
-examples/fileman
-examples/hist_erasedups
-examples/hist_purgecmd
-examples/histexamp
-examples/rl
-examples/rl-callbacktest
-examples/rlbasic
-examples/rlcat
-examples/rlevent
-examples/rltest
-examples/rlversion
-
-libhistory.so.*
-libreadline.so.*
-
-*.dylib
-
-readline.pc
-
-stamp-h