move libuv functions to os.c and io.c module

Despite being an io library, the functions currently implemented with
libuv include some non-I/O tasks like getting the total amount of
memory.

src/io.c

@@ -0,0 +1,26 @@
+/* vi:set ts=8 sts=4 sw=4:
+ *
+ * VIM - Vi IMproved	by Bram Moolenaar
+ *
+ * Do ":help uganda"  in Vim to read copying and usage conditions.
+ * Do ":help credits" in Vim to see a list of people who contributed.
+ * See README.txt for an overview of the Vim source code.
+ */
+
+/*
+ * io.c -- filesystem access, event loop etc.
+ */
+
+#include "vim.h"
+
+#include "uv.h"
+
+int mch_chdir(char *path)
+{
+  if (p_verbose >= 5) {
+    verbose_enter();
+    smsg((char_u *)"chdir(%s)", path);
+    verbose_leave();
+  }
+  return uv_chdir(path);
+}

src/os.c

@@ -0,0 +1,27 @@
+/* vi:set ts=8 sts=4 sw=4:
+ *
+ * VIM - Vi IMproved	by Bram Moolenaar
+ *
+ * Do ":help uganda"  in Vim to read copying and usage conditions.
+ * Do ":help credits" in Vim to see a list of people who contributed.
+ * See README.txt for an overview of the Vim source code.
+ */
+
+/*
+ * os.c -- OS-level calls to query hardware, etc.
+ */
+
+#include "vim.h"
+
+#include "uv.h"
+
+/*
+ * Return total amount of memory available in Kbyte.
+ * Doesn't change when memory has been allocated.
+ */
+long_u mch_total_mem(int special)
+{
+  /* We need to return memory in *Kbytes* but uv_get_total_memory() returns the
+   * number of bytes of total memory. */
+  return uv_get_total_memory() >> 10;
+}

src/os_unix.c

@@ -32,8 +32,6 @@
 
 #include "os_unixx.h"       /* unix includes for os_unix.c only */
 
-#include "uv.h"
-
 #ifdef HAVE_SELINUX
 # include <selinux/selinux.h>
 static int selinux_enabled = -1;
@@ -222,16 +220,6 @@ static struct signalinfo {
   {-1,            "Unknown!", FALSE}
 };
 
-int mch_chdir(char *path)
-{
-  if (p_verbose >= 5) {
-    verbose_enter();
-    smsg((char_u *)"chdir(%s)", path);
-    verbose_leave();
-  }
-  return uv_chdir(path);
-}
-
 /*
  * Write s[len] to the screen.
  */
@@ -333,17 +321,6 @@ int mch_char_avail()         {
   return WaitForChar(0L);
 }
 
-/*
- * Return total amount of memory available in Kbyte.
- * Doesn't change when memory has been allocated.
- */
-long_u mch_total_mem(int special)
-{
-  /* We need to return memory in *Kbytes* but uv_get_total_memory() returns the
-   * number of bytes of total memory. */
-  return uv_get_total_memory() >> 10;
-}
-
 void mch_delay(long msec, int ignoreinput)
 {
   int old_tmode;