docs: convert man page sources to "curldown" format

To render in Git webviews as-is, to make it easier to edit, verify,
and to sync up with curl.

- add options to not build the `.3` man pages:
  - autotools: `--disable-docs`
  - cmake: `LIBSSH2_BUILD_DOCS=OFF`
- building `.3` man pages requires Perl after this patch.
- drop `mansyntax` and the shell / `grep` / GNU `man` tool requirements with it.
- scripts and most logic were copied from curl.
- add `cd2nroff` from curl, with edits to relax curl-specific checks.
- used `nroff2cd` (from curl) to convert from `.3` to `.md`. Then
  manually fixed copyrights, inline function references and a couple
  of other things.

Credits-to: Daniel Stenberg

Ref: eefcc1bda4
Ref: https://github.com/curl/curl/pull/12730
Ref: https://github.com/libssh2/www/issues/25#issuecomment-3289431671

Closes #1660

docs/libssh2_sftp_readdir_ex.md

@@ -0,0 +1,83 @@
+---
+c: Copyright (C) The libssh2 project and its contributors.
+SPDX-License-Identifier: BSD-3-Clause
+Title: libssh2_sftp_readdir_ex
+Section: 3
+Source: libssh2
+See-also:
+  - libssh2_sftp_close_handle(3)
+  - libssh2_sftp_open_ex(3)
+---
+
+# NAME
+
+libssh2_sftp_readdir_ex - read directory data from an SFTP handle
+
+# SYNOPSIS
+
+~~~c
+#include <libssh2.h>
+#include <libssh2_sftp.h>
+
+int
+libssh2_sftp_readdir_ex(LIBSSH2_SFTP_HANDLE *handle,
+                        char *buffer, size_t buffer_maxlen,
+                        char *longentry, size_t longentry_maxlen,
+                        LIBSSH2_SFTP_ATTRIBUTES *attrs);
+~~~
+
+# DESCRIPTION
+
+Reads a block of data from a LIBSSH2_SFTP_HANDLE and returns file entry
+information for the next entry, if any.
+
+*handle* - is the SFTP File Handle as returned by libssh2_sftp_open_ex(3)
+
+*buffer* - is a pointer to a pre-allocated buffer of at least
+*buffer_maxlen* bytes to read data into.
+
+*buffer_maxlen* - is the length of buffer in bytes. If the length of the
+filename is longer than the space provided by buffer_maxlen it will be
+truncated to fit.
+
+*longentry* - is a pointer to a pre-allocated buffer of at least
+*longentry_maxlen* bytes to read data into. The format of the `longname'
+field is unspecified by SFTP protocol. It MUST be suitable for use in the
+output of a directory listing command (in fact, the recommended operation for
+a directory listing command is to display this data).
+
+*longentry_maxlen* - is the length of longentry in bytes. If the length of
+the full directory entry is longer than the space provided by
+*longentry_maxlen* it will be truncated to fit.
+
+*attrs* - is a pointer to LIBSSH2_SFTP_ATTRIBUTES storage to populate
+statbuf style data into.
+
+# RETURN VALUE
+
+Number of bytes actually populated into buffer (not counting the terminating
+zero), or negative on failure. It returns LIBSSH2_ERROR_EAGAIN when it would
+otherwise block. While LIBSSH2_ERROR_EAGAIN is a negative number, it is not
+really a failure per se.
+
+# BUG
+
+Passing in a too small buffer for 'buffer' or 'longentry' when receiving data
+only results in libssh2 1.2.7 or earlier to not copy the entire data amount,
+and it is not possible for the application to tell when it happens!
+
+# ERRORS
+
+*LIBSSH2_ERROR_ALLOC* - An internal memory allocation call failed.
+
+*LIBSSH2_ERROR_SOCKET_SEND* - Unable to send data on socket.
+
+*LIBSSH2_ERROR_SOCKET_TIMEOUT* -
+
+*LIBSSH2_ERROR_SFTP_PROTOCOL* - An invalid SFTP protocol response was
+received on the socket, or an SFTP operation caused an errorcode to be
+returned by the server.
+
+From 1.2.8, LIBSSH2_ERROR_BUFFER_TOO_SMALL is returned if any of the
+given 'buffer' or 'longentry' buffers are too small to fit the requested
+object name.