unlinkapps: avoid deleting too many symlinks

Use `start_with?` to make sure the symlink actually points into one of
the Homebrew directories (depending on given arguments). Previously,
only a substring match was used, which would also remove a symlink to a
hypothetical `/opt/unrelated/usr/local/opt/Unrelated.app`. Even if
unlikely to occur, altering stuff unrelated to Homebrew is bad.

Furthermore, make sure to always use a trailing slash with directories.
Otherwise, e.g., `brew unlinkapps qt` will unlink .app bundles of both
`qt` and `qt5` if both are installed and `brew linkapps qt qt5` was
issued before. (Please ignore that `qt` and `qt5` offer a conflicting
set of .app bundles. This will have to be addressed elsewhere.)

Closes Homebrew/homebrew#45174.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

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