Merge branch 'maint'

Merge branch 'maint'

* maint:
  tests: automatic re-execution works for non-POSIX shells too
  tests: use more POSIX shell features our test scripts

+ Extra non-trivial edits:

* t/ax/test-init.sh: Apply the diffs between past versions of
'defs' in maint and in master; i.e., remove handling of $required
entries 'texi2dvi-o' and 'makeinfo-html', and adjust to the fact
that the parallel testsuite harness is now the default.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>