Make error messages at start-up more consistent. Every line of such

Make error messages at start-up more consistent.  Every line of such
messages now begin with "valgrind: ", and they're more often printed before
the preamble.  This required introducing a new message kind, Vg_FailMsg, and
functions VG_(fmsg) and VG_(fmsg_bad_option), and removing
VG_(err_bad_option).

Where we used to have horrible output like this:

    [ocean:~/grind/ws2] vg5 --tool=massif --threshold=101 date
    ==31877== Massif, a heap profiler
    ==31877== Copyright (C) 2003-2010, and GNU GPL'd, by Nicholas Nethercote
    ==31877== Using Valgrind-3.6.0.SVN and LibVEX; rerun with -h for copyright info
    ==31877== Command: date
    ==31877==
    ==31877== --threshold must be between 0.0 and 100.0
    valgrind: Bad option '--threshold'; aborting.
    valgrind: Use --help for more information.

We now have nice output like this:

    [ocean:~/grind/ws2] vg2 --tool=massif --threshold=101 date
    valgrind: Bad option: --threshold=101
    valgrind: --threshold must be between 0.0 and 100.0
    valgrind: Use --help for more information or consult the user manual.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@11209