[RFAv2 0/3] New option 'set exec-file-mismatch (ask|warn|off)'. Fixes PR gdb/17626.

[Philippe Waroquiers <philippe.waroquiers@skynet.be>]

New option 'set exec-file-mismatch (ask|warn|off)'.  Fixes PR gdb/17626.

This option allows to tell GDB to detect and possibly handle mismatched
exec-files.

A recurrent problem with GDB is that GDB uses the wrong exec-file
when using the attach/detach commands successively.
Also, in case the user specifies a file on the command line but attaches
to the wrong PID, this error is not made visible and gives a not user
understandable behaviour.

RFA 1/3 Implement 'set/show exec-file-mismatch'.
   implements the option.
RFA 2/2 modifies gdb.base/attach.exp to test the option.
RFA 3/3 documents the option in NEWS and in gdb.texinfo

This is the second version of the RFA.
This version handles the comments of Eli about the documentation.
As discussed, the option values have been changed to 'ask|warn|off'
(ask was previouly reload).

Some early comments were given by Tom on the RFC.
Some feedback related to these comments:
  * I have styled the warning/error messages mentionning files in exec.c

  * Related to new lines in the warning message: many other warning
    messages have new lines.  I have kept them, as it looks to me that
    the message is more clear like that.  Of course, I can remove them
    if deemed better without.

  * What happens if the inferior execs another program ?
    This option only controls the behaviour when attaching to a running
    program.  The behaviour of 'exec' is not changed: GDB already properly
    determines the executable to use after exec.