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_keepalive_config.3

@@ -2,12 +2,13 @@
 .SH NAME
 libssh2_keepalive_config - short function description
 .SH SYNOPSIS
-#include <libssh2.h>
 .nf
+#include <libssh2.h>
 
-void libssh2_keepalive_config(LIBSSH2_SESSION *session,
-                              int want_reply,
-                              unsigned interval);
+void
+libssh2_keepalive_config(LIBSSH2_SESSION *session,
+                         int want_reply,
+                         unsigned interval);
 .fi
 .SH DESCRIPTION
 Set how often keepalive messages should be sent. \fBwant_reply\fP indicates