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_userauth_publickey.3

@@ -2,15 +2,17 @@
 .SH NAME
 libssh2_userauth_publickey - authenticate using a callback function
 .SH SYNOPSIS
+.nf
 #include <libssh2.h>
 
-.nf
-int libssh2_userauth_publickey(LIBSSH2_SESSION *session,
-                               const char *user,
-                               const unsigned char *pubkeydata,
-                               size_t pubkeydata_len,
-                               sign_callback,
-                               void **abstract);
+int
+libssh2_userauth_publickey(LIBSSH2_SESSION *session,
+                           const char *user,
+                           const unsigned char *pubkeydata,
+                           size_t pubkeydata_len,
+                           sign_callback,
+                           void **abstract);
+.fi
 .SH DESCRIPTION
 Authenticate with the \fIsign_callback\fP callback that matches the prototype
 below