man: fixups

- add missing `.fi` tags.
- fix misplaced `.nf` tags.
- add `.nf`/`.fi` tags `SYNOPSIS` where missing.
- fix missing/wrong function name from `SH NAME`.
- fix wrong function name in `TH`.
- keep return values in a separate line.
- indent.
- fold long lines.
- deleted `libssh2_channel_direct_streamlocal()`, there is no such function.
- add missing types.
- add missing headers.

Closes #949

docs/libssh2_session_callback_set.3

@@ -5,8 +5,10 @@ libssh2_session_callback_set - set a callback function
 .nf
 #include <libssh2.h>
 
-void *libssh2_session_callback_set(LIBSSH2_SESSION *session,
-                                   int cbtype, void *callback);
+void *
+libssh2_session_callback_set(LIBSSH2_SESSION *session,
+                             int cbtype, void *callback);
+.fi
 .SH DESCRIPTION
 Sets a custom callback handler for a previously initialized session
 object. Callbacks are triggered by the receipt of special packets at the