Add three checkers relevant for examining symbolic links:
- SymlinkTargetEquals checks for a fixed target
- SymlinkTargetContains checks for a substring
- SymlinkTargetMatches checks for a regular expression
They are modelled after FileContentsChecker but implement fewer
variants as symbolic link targets are usually short strings.
Signed-off-by: Zygmunt Krynicki <me@zygoon.pl>