- 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 5db836b2a8Closes#999