unix: Release GIL during all system calls.

Addition of GIL EXIT/ENTER pairs are:

- modos: release the GIL during system calls.  CPython does this as well.

- moduselect: release the GIL during the poll() syscall.  This call can be
  blocking, so it is important to allow other threads to run at this time.

- modusocket: release the GIL during system calls.  Many of these calls can
  be blocking, so it is important to allow other threads to run.

- unix_mphal: release the GIL during the read and write syscalls in
  mp_hal_stdin_rx_chr and mp_hal_stdout_tx_strn.  If we don't do this
  threads are blocked when the REPL or the builtin input function are used.

- file, main, mpconfigport.h: release GIL during syscalls in built-in
  functions that could block.

ports/unix/unix_mphal.c

@@ -31,6 +31,7 @@
 #include <sys/time.h>
 
 #include "py/mphal.h"
+#include "py/mpthread.h"
 #include "py/runtime.h"
 #include "extmod/misc.h"
 
@@ -160,7 +161,9 @@ int mp_hal_stdin_rx_chr(void) {
     } else {
         main_term:;
 #endif
+        MP_THREAD_GIL_EXIT();
         int ret = read(0, &c, 1);
+        MP_THREAD_GIL_ENTER();
         if (ret == 0) {
             c = 4; // EOF, ctrl-D
         } else if (c == '\n') {
@@ -173,7 +176,9 @@ int mp_hal_stdin_rx_chr(void) {
 }
 
 void mp_hal_stdout_tx_strn(const char *str, size_t len) {
+    MP_THREAD_GIL_EXIT();
     int ret = write(1, str, len);
+    MP_THREAD_GIL_ENTER();
     mp_uos_dupterm_tx_strn(str, len);
     (void)ret; // to suppress compiler warning
 }