Add readahead cache to gdb's vFile:pread

Add readahead cache to gdb's vFile:pread

This patch almost halves the time it takes to "target remote + run to
main" on a higher-latency connection.

E.g., I've got a ping time of ~85ms to an x86-64 machine on the gcc
compile farm (almost 2000km away from me), and I'm behind a ~16Mbit
ADSL.  When I connect to a gdbserver debugging itself on that machine
and run to main, it takes almost 55 seconds:

 [palves@gcc76] $ ./gdbserver :9999 ./gdbserver
 [palves@home] $ ssh -L 9999:localhost:9999 gcc76.fsffrance.org
 [palves@home] $ time ./gdb -data-directory=data-directory -ex "tar rem :9999" -ex "b main" -ex "c" -ex "set confirm off" -ex "quit"

 Pristine gdb 7.10.50.20150820-cvs gets us:
 ...
 Remote debugging using :9999
 Reading symbols from target:/home/palves/gdb/build/gdb/gdbserver/gdbserver...done.
 Reading symbols from target:/lib64/ld-linux-x86-64.so.2...(no debugging symbols found)...done.
 0x00007ffff7ddd190 in ?? () from target:/lib64/ld-linux-x86-64.so.2
 Breakpoint 1 at 0x41200c: file ../../../src/gdb/gdbserver/server.c, line 3635.
 Continuing.

 Breakpoint 1, main (argc=1, argv=0x7fffffffe3d8) at ../../../src/gdb/gdbserver/server.c:3635
 3635    ../../../src/gdb/gdbserver/server.c: No such file or directory.
 /home/palves/gdb/build/gdb/gdbserver/gdbserver: No such file or directory.

 real    0m54.803s
 user    0m0.329s
 sys     0m0.064s

While with the readahead cache added by this patch, it drops to:

 real    0m29.462s
 user    0m0.454s
 sys     0m0.054s

I added a few counters to show cache hit/miss, and got:

 readahead cache miss 142
 readahead cache hit 310

Tested on x86_64 Fedora 20.

gdb/ChangeLog:
2015-08-21  Pedro Alves  <palves@redhat.com>

	* remote.c (struct readahead_cache): New.
	(struct remote_state) <readahead_cache>: New field.
	(remote_open_1): Invalidate the cache.
	(readahead_cache_invalidate, readahead_cache_invalidate_fd): New
	functions.
	(remote_hostio_pwrite): Invalidate the readahead cache.
	(remote_hostio_pread): Rename to ...
	(remote_hostio_pread_vFile): ... this.
	(remote_hostio_pread_from_cache): New function.
	(remote_hostio_pread): Reimplement.
	(remote_hostio_close): Invalidate the readahead cache.