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_sign_sk.3

@@ -2,14 +2,16 @@
 .SH NAME
 libssh2_sign_sk - Create a signature from a FIDO2 authenticator.
 .SH SYNOPSIS
-#include <libssh2.h>
 .nf
-int libssh2_sign_sk(LIBSSH2_SESSION *session,
-                    unsigned char **sig,
-                    size_t *sig_len,
-                    const unsigned char *data,
-                    size_t data_len,
-                    void **abstract);
+#include <libssh2.h>
+
+int
+libssh2_sign_sk(LIBSSH2_SESSION *session,
+                unsigned char **sig,
+                size_t *sig_len,
+                const unsigned char *data,
+                size_t data_len,
+                void **abstract);
 
 typedef struct _LIBSSH2_PRIVKEY_SK {
     int algorithm;
@@ -20,7 +22,7 @@ typedef struct _LIBSSH2_PRIVKEY_SK {
     LIBSSH2_USERAUTH_SK_SIGN_FUNC((*sign_callback));
     void **orig_abstract;
 } LIBSSH2_PRIVKEY_SK;
-
+.fi
 .SH DESCRIPTION
 \fIsession\fP - Session instance as returned by
 .BR libssh2_session_init_ex(3)