os_unix: Port mch_FullName and mch_isFullName to libuv.

Basically just delete conditional use of fchdir, since the other called
mch_* functions are already ported to libuv.

src/ex_getln.c

@@ -12,6 +12,7 @@
  */
 
 #include "vim.h"
+#include "os/os.h"
 
 /*
  * Variables shared between getcmdline(), redrawcmdline() and others.