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_channel_process_startup.3

@@ -5,11 +5,13 @@ libssh2_channel_process_startup - request a shell on a channel
 .nf
 #include <libssh2.h>
 
-int libssh2_channel_process_startup(LIBSSH2_CHANNEL *channel,
-                                    const char *request,
-                                    unsigned int request_len,
-                                    const char *message,
-                                    unsigned int message_len);
+int
+libssh2_channel_process_startup(LIBSSH2_CHANNEL *channel,
+                                const char *request,
+                                unsigned int request_len,
+                                const char *message,
+                                unsigned int message_len);
+.fi
 .SH DESCRIPTION
 \fIchannel\fP - Active session channel instance.