Return gdb::optional<std::string> from target_fileio_readlink
This changes to_fileio_readlink and target_fileio_readlink to return a
gdb::optional<std::sring>, and then fixes up the callers and
implementations. This allows the removal of some cleanups.
Regression tested by the buildbot.
gdb/ChangeLog
2018-03-07 Tom Tromey <tom@tromey.com>