Use hash instead of SLIST for versions check.

Use hash table instead of SLIST for storing INDEX file entries whilst
performing version check agains ports tree INDEX file. That speed up
version compare for each installed package.
Add all known verbs to the pkg.8 man page, except 'pkg ssh' because
that's only just been added and is nowhere near ready for prime-time
yet.
Fix spelling
.Ar <foo> is bad style.  Just use .Ar foo
.Ar <foo> bad, .Ar foo good
.Ar <foo> bad, .Ar foo good
.Ar <foo> bad, .Ar foo good