Access executable from remote system when first inferior appears

Access executable from remote system when first inferior appears

This commit modifies remote_add_inferior to take an extra argument
try_open_exec.  If this is nonzero, remote_add_inferior will attempt
to open this inferior's executable as the main executable if no main
executable is open already.  Callers are updated appropriately.

With this commit, remote debugging can now be initiated using only a
"target remote" or "target extended-remote" command; no "set sysroot"
or "file" commands are required, e.g.

  bash$ gdb -q
  (gdb) target remote | gdbserver - /bin/sh
  Remote debugging using | gdbserver - /bin/sh
  Process /bin/sh created; pid = 32166
  stdin/stdout redirected
  Remote debugging using stdio
  Reading symbols from target:/bin/bash...

One testcase required updating as a result of this commit.  The test
checked that GDB's "info files" command does not crash if no main
executable is open, and relied on GDB's inability to access the main
executable over the remote protocol.  The test was updated to inhibit
this new behavior.

gdb/ChangeLog:

	* remote.c (remote_add_inferior): New argument try_open_exec.
	If nonzero, attempt to open the inferior's executable file as
	the main executable if no main executable is open already.
	All callers updated.
	* NEWS: Mention that GDB now supports automatic location and
	retrieval of executable + files from remote targets.

gdb/doc/ChangeLog:

	* gdb.texinfo (Connecting to a Remote Target): Mention that
	GDB can access program files from remote targets that support
	qXfer:exec-file:read and Host I/O packets.

gdb/testsuite/ChangeLog:

	* gdb.server/server-exec-info.exp: Inhibit GDB from accessing
	the main executable over the remote protocol.