Handle error when installing/removing libwinpthread-1.dll gracefully

Use the same technique when installing libwinpthread-1.dll as for many
other DLLs (e.g. gpgex.dll) and use /REBOOTOK when uninstalling it.
libwinpthread-1.dll is a dependency of gpgex.dll and gpgex.dll is
blocked by Windows Explorer as soon as it has been loaded (e.g. after
one has opened the context menu of Windows Explorer). This causes
removing/replacing libwinpthread-1.dll on un-/install to fail and a
reboot is required to complete the un-/installation. (The reboot is/was
already required for replacing gpgex.dll.)

• GnuPG-bug-id: T8215