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_readdir_ex.3

@@ -11,6 +11,7 @@ libssh2_sftp_readdir_ex(LIBSSH2_SFTP_HANDLE *handle,
                         char *buffer, size_t buffer_maxlen,
                         char *longentry, size_t longentry_maxlen,
                         LIBSSH2_SFTP_ATTRIBUTES *attrs);
+.fi
 .SH DESCRIPTION
 Reads a block of data from a LIBSSH2_SFTP_HANDLE and returns file entry
 information for the next entry, if any.