vfs_preopen: introduce support for "preopen:posix-basic-regex = yes"

vfs_preopen: introduce support for "preopen:posix-basic-regex = yes"

This will allow the usage of patterns as
'POSIX Basic Regular Expression'

      vfs objects = preopen
      preopen:posix-basic-regex = yes
      preopen:names = /Re7599Ex\([0-9]\).*\.txt/test\([0-9]*\)\.dat/

The key is that exactly one 'subexpression' starting with '\(' and
ending with '\)' is specified in order to select the position where
the digits are searched.

E.g. given a file name 'Re7599Ex01234.txt' will actually preopen:

  Re7599Ex01234.txt
  Re7599Ex11234.txt
  Re7599Ex21234.txt
  Re7599Ex31234.txt
  Re7599Ex41234.txt

As '\([0-9]\)' will only match the first digit after 'Re7599Ex'.

It also means it's now possible to have digits in the fixed part of the
filename, which was the actual motivation for this patchset.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>