user-accounts: Disallow whitespace-only names

This prevents users from creating an account with a full name consisting
solely of whitespace, e.g., space and tab characters. Having "invisible"
real names can cause confusion on the login screen.

v2: Check for all UTF-8 "space" characters (review from Bastien Nocera)
v3: Correctly iterate over multi-byte chars (review from Bastien Nocera)

https://bugzilla.gnome.org/show_bug.cgi?id=699940
user-accounts: Validate real name on change

This prevents users from setting invalid real names for accounts by
editing the name from the user panel. This applies the same check as is
done when first adding the account.

https://bugzilla.gnome.org/show_bug.cgi?id=699940