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_sftp_realpath.3

@@ -4,10 +4,14 @@ libssh2_sftp_realpath - convenience macro for \fIlibssh2_sftp_symlink_ex(3)\fP
 .SH SYNOPSIS
 .nf
 #include <libssh2.h>
+#include <libssh2_sftp.h>
 
 #define libssh2_sftp_realpath(sftp, path, target, maxlen) \\
-  libssh2_sftp_symlink_ex((sftp), (path), strlen(path), (target), (maxlen), \\
-                          LIBSSH2_SFTP_REALPATH)
+    libssh2_sftp_symlink_ex((sftp), \\
+                            (path), strlen(path), \\
+                            (target), (maxlen), \\
+                            LIBSSH2_SFTP_REALPATH)
+.fi
 .SH DESCRIPTION
 This is a macro defined in a public libssh2 header file that is using the
 underlying function \fIlibssh2_sftp_symlink_ex(3)\fP.