file: Use 'long' to represent the parsed LOCK TIMEOUT value

Convert the StringToInt helper into a StringToLong helper with a 'long'
result type.  This will make the helper more useful to other callers
that want to use strtol.

While at it, also check errno after calling strtol in case the
conversion fails with a range error.

Source/cmSystemTools.h

@@ -458,8 +458,8 @@ public:
   static std::vector<std::string> tokenize(const std::string& str,
                                            const std::string& sep);
 
-  /** Convert string to int. Expected that the whole string is an integer */
-  static bool StringToInt(const char* str, int* value);
+  /** Convert string to long. Expected that the whole string is an integer */
+  static bool StringToLong(const char* str, long* value);
 
 #ifdef _WIN32
   struct WindowsFileRetry