Add support for VxWorks 5.x, 6.x and 653

[Joel Brobecker <brobecker@adacore.com>]

This is mostly a rebase of the previous series, submitted almost
a year ago (already!). We adjusted some of the patches based on
the feedback that we received so far.  We also made some corrections
for issues that we discovered since.  I've also done another pass
at trying to fix as many obvious coding style issues as I could see.

I have also added full documentation, as well as NEWS entry
(there are two patches with documentation: #04/18, and #18/18).

Please note the introduction of patch #5 since the first series:
    new struct bp_target_info target_private_data field
(see patch itself for more info about it)

Pending comments, I would like to commit these patches after
a reasonable wait (say, around the end of next week, for instance).