tidy-up: C header use

- drop unused or duplicate C headers.
- add missing ones (that worked by chance).
  (`string.h`, `stdlib.h`)
- mention the functions that need certain headers.
- move some headers from crypto header to crypto C source.
- reorder headers in some places.
- simplify the #if tree for `sys/select.h` in `libssh2_priv.h`.
- move scp-specific macros next to their header to `scp.c`
  Follow-up to 5db836b2a8

Closes #999

src/misc.c

@@ -40,21 +40,18 @@
 #include "libssh2_priv.h"
 #include "misc.h"
 
-#include <stdlib.h>
-
 #ifdef HAVE_UNISTD_H
 #include <unistd.h>
 #endif
 
+#include <errno.h>
+
 #ifdef WIN32
 /* Force parameter type. */
 #define recv(s, b, l, f)  recv((s), (b), (int)(l), (f))
 #define send(s, b, l, f)  send((s), (b), (int)(l), (f))
 #endif
 
-#include <stdio.h>
-#include <errno.h>
-
 /* snprintf not in Visual Studio CRT and _snprintf dangerously incompatible.
    We provide a safe wrapper if snprintf not found */
 #ifdef LIBSSH2_SNPRINTF