libpkg: bugfix with '%t'  - Also, pkg info: enhancement request.

fix to endless loop when using %{...%} with %t timestamp option - this was simply due to the corresponding .has_trailer option being errornously set to false.

2) Enhancement request. Although the installation timestamp can be retrieved with 'pkg query', there is no current way to see this information with 'pkg info'. This patch adds this functionality to the 'full' output of 'pkg info' (NB This requires the above bugfix to be applied!)

Hi. The lack of 'installation date' in 'pkg info' was bugging me, so I produced this patch.

Comments welcome,

Cheers Jamie