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/sftp_nonblock.c

@@ -18,9 +18,6 @@
 #ifdef HAVE_SYS_SOCKET_H
 #include <sys/socket.h>
 #endif
-#ifdef HAVE_SYS_SELECT_H
-#include <sys/select.h>
-#endif
 #ifdef HAVE_UNISTD_H
 #include <unistd.h>
 #endif
@@ -34,11 +31,7 @@
 #include <sys/time.h>
 #endif
 
-#include <sys/types.h>
-#include <fcntl.h>
-#include <errno.h>
 #include <stdio.h>
-#include <ctype.h>
 
 static const char *pubkey = "/home/username/.ssh/id_rsa.pub";
 static const char *privkey = "/home/username/.ssh/id_rsa";