[RFC 00/17] Merge event loop implementations

[Tom Tromey]

This series merges the gdb and gdbserver forks of event-loop.[ch].

This is an RFC because there are a few possibly unresolved issues.

Most of the patches are straightforward.  The series begins by tidying
up the gdb event-loop code, removing things that are specific to gdb.
Then, the code is moved.  After this, gdbserver is switched to use the
common code; and finally, a few cleanups are applied.

I initially attempted something more ambitious here: unifying the
async event and async signal code in event-loop (as I do not
understand the reason for the difference); and then further handling
the async signal code using the same code path as ordinary file
descriptors.

However, this didn't work, and since it was not directly important to
my goal of merging event loops, I dropped it.  I think it may be worth
reviving.  For example I think th async event code suffers from the
same race that led Pedro to change the async signal code to use a the
self-pipe trick.

Another related possible to-do item is changing the ser-event code to
maintain just a single self-pipe.  It seems to me that there's never a
reason to need more than one.

create_file_handler may have a latent bug where the global select
masks are not updated if it is called a second time for the same file
descriptor.  Both versions of the event loop have this issue; I didn't
try to verify it, so perhaps I'm just misunderstanding the code here.

The final patch simplifies the rather convoluted handling of "serial"
(meaning remote protocol) input in gdbserver.  It passes testing, but
I wonder whether there's some subtle reason that the code is written
the way it is.  This is one of the unresolved issues I mentioned.

The second unresolved issue involves the USE_WIN32API code.  Before
this series, gdbserver used gdb_fildes_t, defined like:

    #if USE_WIN32API
    #include <winsock2.h>
    typedef SOCKET gdb_fildes_t;
    #else
    typedef int gdb_fildes_t;
    #endif

gdb did not use this approach, but does have a separate gdb_select
implementation in mingw-hdep.c, which gdbserver does not.

I don't know much about Windows, so I don't know why these things are
needed.  I did a build using " --host=i686-w64-mingw32
--target=i686-w64-mingw32", and everything built just fine using a
POSIX-style API.

Given that, I removed gdb_fildes_t in this series.  However, perhaps
it is still needed and this series needs some more work.  I could use
some advice here -- when is this code actually needed and is there a
way I can reproduce any problems?  I don't have a Windows host, so I'm
hoping for some sort of compile-time error using a mingw cross.

Regression tested by the buildbot.

Tom