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.
config.vm.box_url can use local files

Updated the documentation to show that box_url can use "file://". This ability was added via in issue #1087 that was added 2 years ago.

https://github.com/mitchellh/vagrant/pull/1087
add rsync_install capability to tinycore guest
Merge pull request #3178 from cuberri/fix/master/tinycore-rsync_install

guests/tinycore: add rsync_install capability to tinycore guest
Update CHANGELOG
Merge pull request #3174 from freshteapot/3173

website/docs: config.vm.box_url can use local files
website/docs: clean up example
website/docs: doc windows gotcha [GH-3170]
website/docs: note that rsync__args needs to be array [GH-3168]
Merge pull request #3167 from xraj/bugfix/linux-nfs-exports-regex

hosts/linux: Fix Linux NFS exports pruning due to bad `sed` expression
Update CHANGELOG
synced_folders/rsync: execute rsync_post cap if it exists [GH-3163]
synced_folders/rsync: don't watch excluded paths [GH-3159]
core: Execute PowerShell scripts even with spaces in dir [GH-3100]
providers/virtualbox: import progress works on Windows
provisioners/salt: style nitpicks
core: error earlier if adding a path that doesn't exist
Error message if tampering with plugins.json