Fix CreateProcess call of GpgEX

* src/client.cc (default_uiserver_cmdline): Renamed to
default_uiserver_name.
* src/exechelp.c (gpgex_spawn_detached): Split command line
and program name.

This is the old "c:\program.exe" issue that Windows tries to
find a Program based on some heuristics if only the commandline
is given and not the program name.

• GnuPG-Bug-Id: T5272