- 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
Functions that return an "int", and friends, return LIBSSH2SFTP_EAGAIN.
Functions that return a structure return NULL and set the error to
LIBSSH2_ERROR_EAGAIN
