add option for username stripping

[atemp]

Many people use space to wakeup display from screensaver, so it happens that they end up having one space in front of their username in greeter. They will then get an incorrect password message even and trying it again won't help, because they only reenter their password instead of double checking the username. One leading space is hard to spot, since the password field at least looks to have more space before the first circle.

This pull request adds a config option username-strip-whitespace.

[ochosi]

While this sounds like a usability improvement I would personally not make this an option. Either the greeter behaves this way or not.
I don't think having a space in the username is even possible, so this should just happen unconditionally by default.

[atemp]

no problem, that makes PR smaller and i agree, that it should never hurt

[bluesabre]

Nice and tidy. Thanks for the patch and discussions!