Hang at start due to GetUserNameEx

[FauxFaux]

A user reports that they get 1+ minute hangs at startup. Debugging v013 reveals:

   0  Id: 14c0.cbc Suspend: 1 Teb: 000007ff`fffde000 Unfrozen
*** ERROR: Symbol file could not be found.  Defaulted to export symbols for C:\Windows\system32\RPCRT4.dll - 
Child-SP          RetAddr           Call Site
00000000`0013f298 000007fe`fe82aa76 ntdll!ZwAlpcSendWaitReceivePort+0xa
00000000`0013f2a0 000007fe`fe8ccb64 RPCRT4!NdrDllCanUnloadNow+0x31c6
00000000`0013f360 000007fe`fe8ccd55 RPCRT4!Ndr64AsyncClientCall+0xe04
*** ERROR: Symbol file could not be found.  Defaulted to export symbols for C:\Windows\system32\SSPICLI.DLL - 
00000000`0013f620 000007fe`fd7c1360 RPCRT4!NdrClientCall3+0xf5
00000000`0013f9b0 000007fe`fd7c1214 SSPICLI!GetUserNameExW+0x18c
00000000`0013fa70 000007fe`fd7c9a4f SSPICLI!GetUserNameExW+0x40
00000000`0013fac0 00000001`3f381d3a SSPICLI!GetUserNameExA+0x6f
00000000`0013fb10 00000001`3f32691b putty!get_username+0x66 [c:\scm\putty\windows\winmisc.c @ 76]
00000000`0013fb40 00000001`3f3759a1 putty!setup_config_box+0x2363 [c:\scm\putty\config.c @ 1887]
00000000`0013fc80 00000001`3f3771a4 putty!do_config+0x25 [c:\scm\putty\windows\windlg.c @ 643]
00000000`0013fcb0 00000001`3f392590 putty!WinMain+0x78c [c:\scm\putty\windows\window.c @ 659]

..that is, that GetUserNameEx is Slow.

This is only called to make a small UI enhancement, so seems worth removing.

[FauxFaux]

I've just entirely ripped this off, I can't find any way to make it Fast.