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_last_error.3

@@ -2,12 +2,13 @@
 .SH NAME
 libssh2_sftp_last_error - return the last SFTP-specific error code
 .SH SYNOPSIS
+.nf
 #include <libssh2.h>
 #include <libssh2_sftp.h>
 
 unsigned long
 libssh2_sftp_last_error(LIBSSH2_SFTP *sftp);
-
+.fi
 .SH DESCRIPTION
 \fIsftp\fP - SFTP instance as returned by
 .BR libssh2_sftp_init(3)