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_userauth_password_ex.3

@@ -2,19 +2,22 @@
 .SH NAME
 libssh2_userauth_password_ex - authenticate a session with username and password
 .SH SYNOPSIS
-#include <libssh2.h>
 .nf
-int libssh2_userauth_password_ex(LIBSSH2_SESSION *session,
-                    const char *username,
-                    unsigned int username_len,
-                    const char *password,
-                    unsigned int password_len,
-                    LIBSSH2_PASSWD_CHANGEREQ_FUNC((*passwd_change_cb)));
+#include <libssh2.h>
+
+int
+libssh2_userauth_password_ex(LIBSSH2_SESSION *session,
+                             const char *username,
+                             unsigned int username_len,
+                             const char *password,
+                             unsigned int password_len,
+                           LIBSSH2_PASSWD_CHANGEREQ_FUNC((*passwd_change_cb)));
 
 #define libssh2_userauth_password(session, username, password) \\
- libssh2_userauth_password_ex((session), (username), \\
-                              strlen(username), \\
-                              (password), strlen(password), NULL)
+     libssh2_userauth_password_ex((session), (username), \\
+                                  strlen(username), \\
+                                  (password), strlen(password), NULL)
+.fi
 .SH DESCRIPTION
 \fIsession\fP - Session instance as returned by
 .BR libssh2_session_init_ex(3)