- 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.
As the long-term goal is to get rid of the extensive set of
macros from the API we can just as well start small by not adding
new macros when we add new functions. Therefore we let the
function be libssh2_sftp_statvfs() plainly without using an _ex
suffix.
I also made it use size_t instead of unsigned int for the string
length as that too is a long-term goal for the API.
