MEDIUM: pattern: Add support for virtual and optional files for patterns

MEDIUM: pattern: Add support for virtual and optional files for patterns

Before this patch, it was not possible to use a list of patterns, map or a
list of acls, without an existing file.  However, it could be handy to just
use an ID, with no file on the disk. It is pretty useful for everyone
managing dynamically these lists. It could also be handy to try to load a
list from a file if it exists without failing if not. This way, it could be
possible to make a cold start without any file (instead of empty file),
dynamically add and del patterns, dump the list to the file periodically to
reuse it on reload (via an external process).

In this patch, we uses some prefixes to be able to use virtual or optional
files.

The default case remains unchanged. regular files are used. A filename, with
no prefix, is used as reference, and it must exist on the disk. With the
prefix "file@", the same is performed. Internally this prefix is
skipped. Thus the same file, with ou without "file@" prefix, references the
same list of patterns.

To use a virtual map, "virt@" prefix must be used. No file is read, even if
the following name looks like a file. It is just an ID. The prefix is part
of ID and must always be used.

To use a optional file, ie a file that may or may not exist on a disk at
startup, "opt@" prefix must be used. If the file exists, its content is
loaded. But HAProxy doesn't complain if not. The prefix is not part of
ID. For a given file, optional files and regular files reference the same
list of patterns.

This patch should fix the issue #2202.