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_knownhost_check.3

@@ -5,13 +5,16 @@
 .SH NAME
 libssh2_knownhost_check - check a host+key against the list of known hosts
 .SH SYNOPSIS
+.nf
 #include <libssh2.h>
 
-int libssh2_knownhost_check(LIBSSH2_KNOWNHOSTS *hosts,
-                            const char *host,
-                            const char *key, size_t keylen,
-                            int typemask,
-                            struct libssh2_knownhost **knownhost);
+int
+libssh2_knownhost_check(LIBSSH2_KNOWNHOSTS *hosts,
+                        const char *host,
+                        const char *key, size_t keylen,
+                        int typemask,
+                        struct libssh2_knownhost **knownhost);
+.fi
 .SH DESCRIPTION
 Checks a host and its associated key against the collection of known hosts,
 and returns info back about the (partially) matched entry.