Formats the network peer user agent text (or clean subversion) by removing the forward slash prefix and suffix. Example: /Satoshi:0.11.2/ --> Satoshi:0.11.2

The forward slashes in the User Agent text are occupying two spaces and seem unnecessary for the Qt wallet UI.
This code is forked from my original pull for Dark Silk: https://github.com/SCDeveloper/DarkSilk-Release-Candidate/pull/138
