Windows consul command line paths

#### Overview of the Issue

Consul does not understand a windows path correctly when it ends with a `\` as is common for folders. `For example using consul.exe agent -config-dir "C:\Program Files\Consul\consul.d\"` will fail with `==> config: Open failed on C:\Program Files\Consul\consul.d". open C:\Program Files\Consul\consul.d": The filename, directory name, or volume label syntax is incorrect.` notice the error message leaving out the trailing `\`

The same problem can be seen on other config settings also, for example on data_dir.

#### Reproduction Steps

Steps to reproduce this issue, eg:

1. Try to run consul with `consul.exe agent -config-dir "C:\Program Files\Consul\consul.d\" `and config files placed in C:\Program Files\Consul\consul.d\

### Operating system and Environment details

Windows server 2019
Consul v1.10.1
Revision db839f18b

Start by reproducing the Windows command with a trailing backslash in -config-dir, then trace how Consul parses and opens config paths. Check the same behavior for data_dir. Done means trailing-backslash Windows paths work without dropping the slash or producing an invalid path error.

Written by the indexing model from the issue text.