Bug#833674: apt: Use ptsname_r if available

[Esa Peuha <esa.peuha@gmail.com>]

Source: apt

Commit 0a81c22 removed the test for ptsname_r from configure.ac but
there is a code block in apt-pkg/deb/dpkgpm.cc that is trying to use
ptsname_r if it exists. Although that code block is conditional inside
"#ifdef HAVE_PTS_NAME_R" while the configure test was trying to define
HAVE_PTSNAME_R so I don't think that has actually worked at all, but I
still think it would be better to put the configure test back and make
it work properly.