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_agent_get_identity_path.3

@@ -5,10 +5,12 @@
 .SH NAME
 libssh2_agent_get_identity_path - gets the custom ssh-agent socket path
 .SH SYNOPSIS
+.nf
 #include <libssh2.h>
 
 const char *
 libssh2_agent_get_identity_path(LIBSSH2_AGENT *agent);
+.fi
 .SH DESCRIPTION
 Returns the custom agent identity socket path if set using libssh2_agent_set_identity_path()