Merge fixes and features from #430096

This one is specific to 6.x-2.x only
search_directories.module:1.1.2.4
Fixed broken search by consequently renaming table and not renaming type; the inconsistent use of user visible item naming, directory layout, function-, table- and record prefixes is a mess that still needs cleanup.

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I merged all changes

1.21
Fixed typo.
-> issue does not exist or has already been fixed in DRUPAL-6--2

1.22
Fixed configuration of "Search Label" must use #default_value not #value.
-> issue does not exist or has already been fixed in DRUPAL-6--2

1.23
Fixed superfluous debugging output.
-> issue does not exist or has already been fixed in DRUPAL-6--2

1.24
Added code to clear associated data from search_dataset, search_files_files and search_files_directories tables when manually deleting a directory. Create and leverage function search_files_delete_content_by_directory_fullpath().
-> merged as search_directories.module:1.1.2.5

1.25
Fixed functional gap where vanished files were not handled at all. Regularly compare database to filesystem and remove vanished files from database. Create and leverage function search_files_delete_content_by_file_fullpath().
-> merged as search_directories.module:1.1.2.6

1.26
Fixed issue with files vanished before being indexed. Skip over them, otherwise helper applications will get passed a nonexistent file name and fail.
-> merged as search_directories.module:1.1.2.7

1.27
Fixed issue with files vanished after being indexed. Skip over them, otherwise otherwise functions like filectime() will get passed a nonexistent file name and fail.
-> merged as search_directories.module:1.1.2.8

1.28
Added support for relative pathes. Changed handling of trailing slash to be more user friendly.
-> merged as search_directories.module:1.1.2.9

1.29
Changed hardcoded "Directory Rescan Age" into a configurable parameter
-> merged as search_directories.module:1.1.2.10

1.30
Added detection for PHP safe_mode and skip over shell_exec() which would fail anyway.
-> merged as search_files.module:1.19.2.5
-> merged as search_directories.module:1.1.2.11

1.31
Added dashboard as default local task to search_files menu to allow some status insight.
-> merged as search_files.module:1.19.2.6