Fix compilation with VS2017

The primary issue is that in MSVC 14.00 (VS2015) Microsoft added
snprintf as a function to the standard library and prevents users from
defining it to something else (typically, this was _snprintf). So, only
define it when using _MSC_VER < 1900.

Other changes are:
- Fix macro definition of dup2
- Add macro for getpid
- Add function definition for win32_dup
- Add missing EXTERNs

Signed-off-by: Tim Crawford <crawfxrd@gmail.com>

include/iscsi.h

@@ -166,7 +166,7 @@ enum iscsi_immediate_data {
 	ISCSI_IMMEDIATE_DATA_NO  = 0,
 	ISCSI_IMMEDIATE_DATA_YES = 1
 };
-int iscsi_set_immediate_data(struct iscsi_context *iscsi, enum iscsi_immediate_data immediate_data);
+EXTERN int iscsi_set_immediate_data(struct iscsi_context *iscsi, enum iscsi_immediate_data immediate_data);
 
 /*
  * This function is used to set the desired mode for initial_r2t
@@ -179,7 +179,7 @@ enum iscsi_initial_r2t {
 	ISCSI_INITIAL_R2T_NO  = 0,
 	ISCSI_INITIAL_R2T_YES = 1
 };
-int
+EXTERN int
 iscsi_set_initial_r2t(struct iscsi_context *iscsi, enum iscsi_initial_r2t initial_r2t);