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_session_init_ex.3

@@ -2,14 +2,18 @@
 .SH NAME
 libssh2_session_init_ex - initializes an SSH session object
 .SH SYNOPSIS
+.nf
 #include <libssh2.h>
 
 LIBSSH2_SESSION *
-libssh2_session_init_ex(LIBSSH2_ALLOC_FUNC((*myalloc)), LIBSSH2_FREE_FUNC((*myfree)), LIBSSH2_REALLOC_FUNC((*myrealloc)), void *abstract);
+libssh2_session_init_ex(LIBSSH2_ALLOC_FUNC((*myalloc)),
+                        LIBSSH2_FREE_FUNC((*myfree)),
+                        LIBSSH2_REALLOC_FUNC((*myrealloc)),
+                        void *abstract);
 
 LIBSSH2_SESSION *
 libssh2_session_init(void);
-
+.fi
 .SH DESCRIPTION
 \fImyalloc\fP - Custom allocator function. Refer to the section on Callbacks
 for implementing an allocator callback. Pass a value of NULL to use the