win: Append process dir to $PATH

This allows executables to be found by :!, system(), and executable() if
they live next to ("sibling" to) nvim.exe. This is what gvim on Windows
does, and also matches the behavior of Win32 SearchPath().

c4a249a736/src/os_win32.c (L354-L370)

test/functional/options/defaults_spec.lua

@@ -96,6 +96,10 @@ describe('startup defaults', function()
       eq(meths.get_option('runtimepath'), meths.get_option('packpath'))
     end)
   end)
+
+  it('v:progpath is set to the absolute path', function()
+    eq(eval("fnamemodify(v:progpath, ':p')"), eval('v:progpath'))
+  end)
 end)
 
 describe('XDG-based defaults', function()