Support records larger than 32kb in 'apt show'

The default buffer size for pkgTagFile is 32kb which should be big
enough for everything… expect for enormous lists of provides,
resulting in:

$ apt show librust-winapi-dev
E: Unable to parse package file /var/lib/apt/lists/ftp.br.debian.org_debian_dists_unstable_main_binary-amd64_Packages (2)
E: Internal Error, Unable to parse a package record

The "apt-cache show" codepath uses instead a max size for all files,
which seems a bit excessive, but works – using the max size for the file
in question seems most appropriate.

The patch is written for the 1.6.y series as a rewrite of the related
code in the 1.7.y series (commit bf53f39c)
removed this problem before it was reported.

Closes: #905527
LP: #1787120