coredump,basic: generalize O_TMPFILE handling a bit

coredump,basic: generalize O_TMPFILE handling a bit

This moves the O_TMPFILE handling from the coredumping code into common library
code, and generalizes it as open_tmpfile_linkable() + link_tmpfile(). The
existing open_tmpfile() function (which creates an unlinked temporary file that
cannot be linked into the fs) is renamed to open_tmpfile_unlinkable(), to make
the distinction clear. Thus, code may now choose between:

 a) open_tmpfile_linkable() + link_tmpfile()
 b) open_tmpfile_unlinkable()

Depending on whether they want a file that may be linked back into the fs later
on or not.

In a later commit we should probably convert fopen_temporary() to make use of
open_tmpfile_linkable().

Followup for: #3065