posix: Sync tempname with gnulib [BZ #26648]

It syncs with gnulib commit b1268f22f443e8e4b9e.  The try_tempname_len
now uses getrandom on each iteration to get entropy and only uses the
clock plus ASLR as source of entropy if getrandom fails.

Checked on x86_64-linux-gnu and i686-linux-gnu.