Fix Issue #8298, Don't throw exceptions on broken symlinks.

Using dirEntries to loop trough all the files/directories in a directory
will curently throw an Exception when encountering an broken symlink.

Whit this change DirEntry will no longer throw an Exception for broken
symlinks, but in that case isDir and isFile will return flase while
isSymlink returns true.
[trivial] remvoe trailing whitespace
fix issue 11808
adjust documentation to reflect new capabilities
fix issue 11839
add _ to allowed characters in regex identifier
fix issue 11350

Do not throw on bad UTF inside of a C callback
fix issue 8203, similar issue with lookaround

When using a temporary engine as closure its generation counter
should be tracked separately for each lookaround.

For now just use built-in AA, later we could find better places
to store this counter in.
Merge pull request #1841 from blackwhale/issue-8203

fix issue 8203, similar issue with lookaround
Merge pull request #1838 from blackwhale/issue-11839

Fix issue 11839
Merge pull request #1837 from blackwhale/issue-11808

Fix issue 11808 std.uni.CodepointSet('А', 'Я'+1, 'а', 'я'+1) asserts
Merge pull request #1142 from MikeWey/Issue8298

Fix Issue #8298, Don't throw exceptions on broken symlinks.
Merge pull request #1842 from blackwhale/issue-11350

Fix issue 11350 ibphobos2 regex match segfaults when a rare HTTP header is received