Allow ordering of profile scripts.

Allow ordering of profile scripts.

If "# profile-required: X" appears in a file under .profile.d then
ensure that X (which may be a list) is processed before the script in
question.
Always source OS.bashrc, PATH.bashrc and BECOME.bashrc before everything
else, if they exist.
Use this functionality to ensure that p4.bashrc isn't sourced before
vim.bashrc so that P4EDITOR can be set.