- simplify `.TH` headers.
- delete empty lines before sections.
- update template with an `AVAILABILITY` section.
Left libssh2 version number in the `.TH` header for entries without an
`AVAILABILITY` section, or where there was a different version number
there.
- 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
