Cygwin: fix return value of symlink_info::check

Cygwin: fix return value of symlink_info::check

Currently it is possible for symlink_info::check to return -1 in case
we're searching for foo and find foo.lnk that is not a Cygwin symlink.
This contradicts the new meaning attached to a negative return value
in commit 19d59ce75d.  Fix this by setting "res" to 0 at the beginning
of the main loop and not seting it to -1 later.

Also fix the commentary preceding the function definition to reflect
the current behavior.

Addresses: https://cygwin.com/pipermail/cygwin/2022-August/252030.html