plat, vendor: systematically escape net and file names in Tcl.

plat, vendor: systematically escape net and file names in Tcl.

Before this commit, there was only occasional quoting of some names
used in any Tcl files. (I'm not sure what I was thinking.)

After this commit, any substs that may include Tcl special characters
are escaped. This does not include build names (which are explicitly
restricted to ASCII to avoid this problem), or attribute names (which
are chosen from a predefined set). Ideally we'd use a more principled
approach but Jinja2 does not support custom escaping mechanisms.

Note that Vivado restricts clock names to a more restrictive set that
forbids using Tcl special characters even when escaped.

Fixes #375.