apt list: Fix behavior of regex vs fnmatch vs wildcards

Previously (and still in cacheset), patterns where only allowed to
start with ? or ~, which ignores the fact that a pattern might just
as well start with a negation, such a !~nfoo.

Also, we ignored the --regex flag if it looked like this, which
was somewhat bad.

Let's change this all:

* If --regex is given, arguments are always interpreted as regex
* If it is a valid package wildcard (name or * characters), then
  it will be interpreted as a wildcard - this set of characters is
  free from meaningful overlap with patterns.
* Otherwise, the argument is interpreted as a pattern.

For a future version, we need to adapt parsing for cacheset and
list to use a common parser, to avoid differences in their
interpretation. Likely, this code will go into the pattern parser,
such that it generates a pattern given a valid fnmatch argument
for example.

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