ocamlbuild: get myocamlbuild.ml tags from a new -plugin-tag(s) option instead of _tags

The previous approach to use _tags to get tags to compile
myocamlbuild.ml had one irritating downside: it would also take the
tags of the (true: foo) lines, which are certainly not intended by the
user to be applied to plugin compilation as well.

These additional tags looked mostly harmless. Of course, it turns out
that there is one case where they're not: as the plugin compilation
command already links "unix.cma" with the plugin, a user having
`true: use_unix` or `true: package(unix)` in its _tags file would get
a plugin compilation error due to double-linking of unix.cmxa. This
caused a regression breaking build in some projects, which is not
acceptable.

The current approach of using a specific command-line option is a bit
more annoying for end-users (you have to retype it each time, or script
ocamlbuild invocation from somewhere else), so we expect it to get
less widely used. It is still interesting for OASIS for example, or
people already using a convenience wrapper (eg. corebuild).

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14035 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02

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