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

example/ssh2_agent.c

@@ -22,12 +22,8 @@
 #include <arpa/inet.h>
 #endif
 
-#include <sys/types.h>
-#include <fcntl.h>
-#include <errno.h>
 #include <stdio.h>
-#include <stdlib.h>
-#include <ctype.h>
+#include <string.h>
 
 static const char *username = "username";