Vagrant 1.5: Fix Linux NFS exports pruning due to bad `sed` expression

The mount id is a file path which will contain forward slashes. A
previous attempt (although notably missing in the Linux host plugin) at
fixing this used `String.gsub` to escape the forward slashes; however,
the solution that eventually made its way into the 1.5 release uses
`Regexp.escape` which doesn't escape forward slashes.

The Ruby `Regexp.escape` method does not escape forward slashes because
they are not RE meta-characters; their special meaning is specific to
sed expressions as delimiters. To avoid the issue entirely, we can use
an alternative delimiter by prefixing the address expression with a
backslash with the desired delimiter character following.

Use control character (ASCII code point `0x01`) as expression delimiter
so it is very unlikely an identifier will have a conflicting character
within it.