[RFC][PATCH 0/2][PR cli/34409] Fix SIGTTIN due to readline macroinput.

The two patches fix (gdb:cli/34409): [gdb receives SIGTTIN when user uses readline macros]

It adds support of any kind of readline macros: the one defined by the user in his ~/.inputrc 
or the one, for example, recorded at a debug scene with Cx-( ... Cx-) Cx-e -- into gdb.

[2] I managed to do so by a slight extension of the current event-loop over file 
    descriptors. Now, apart from direct events like the ones from poll api, 
    file descriptors also can specify that they have a `synthetic input` 
    pending associated with them.

[1] It was easy to integrate this new behavior into select round robin, but not poll.
    So I reworked the poll's round robin so that it looks like select's version now.

I ran the test-suite via `make check -j$(nproc)` on x86-64 Linux under `guix shell -D gdb clang-toolchain`.
There were unexpected failures fluctuating around 456 in gdb, but no visible regressions between 
the original master and my patches on top of it.

I have not previously completed any FSF copyright assignment
This is my first contribution to the project.

kurku (2):
  gdb: event-loop: Rework poll's version of round robin
  gdb: Fix readline macros hazards.

 gdb/event-top.c          |  35 +++++++++++
 gdb/ui.h                 |   3 +
 gdbsupport/event-loop.cc | 131 +++++++++++++++++++++++++++++----------
 gdbsupport/event-loop.h  |   4 ++
 4 files changed, 139 insertions(+), 34 deletions(-)

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2.43.0