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/comp.c

@@ -37,6 +37,7 @@
  */
 
 #include "libssh2_priv.h"
+
 #ifdef LIBSSH2_HAVE_ZLIB
 #include <zlib.h>
 #undef compress /* dodge name clash with ZLIB macro */