Make foreign imports of realpath and utimensat safe

Although these functions never return EINTR, signals may still affect
these calls.  In particular, realpath on Mac OS X contains a
retry-if-interrupted loop on statfs64.  If the file system is slow
(e.g. SSHFS), the call will hang due to the barrage of periodic alarm
signals from the GHC runtime.  Marking the foreign import as safe
appears to silence the alarm signals.

We also do this for utimensat as a precautionary measure.  Given that
these are system calls that manipulate the file system, the performance
overhead should be negligible.

Fixes haskell#35.

System/Directory/Internal/C_utimensat.hsc

@@ -42,7 +42,7 @@ toCTimeSpec t = CTimeSpec (CTime sec) (truncate $ 10 ^ (9 :: Int) * frac)
     (sec,  frac)  = if frac' < 0 then (sec' - 1, frac' + 1) else (sec', frac')
     (sec', frac') = properFraction (toRational t)
 
-foreign import ccall unsafe "utimensat" c_utimensat
+foreign import ccall "utimensat" c_utimensat
   :: CInt -> CString -> Ptr CTimeSpec -> CInt -> IO CInt
 
 #endif

System/Directory/Internal/Posix.hsc

@@ -25,7 +25,7 @@ c_PATH_MAX | c_PATH_MAX' > toInteger maxValue = Nothing
 c_PATH_MAX = Nothing
 #endif
 
-foreign import ccall unsafe "realpath" c_realpath
+foreign import ccall "realpath" c_realpath
   :: CString -> CString -> IO CString
 
 withRealpath :: CString -> (CString -> IO a) -> IO a