fix(ignore, fs): allow loading ignore patterns behind symlink (fixes #10785) (#10786)

The recent change to disallow following symlinks by default conflicts
with the expected behavior of .stignore. This adds a new flag which may
be passed to OpenFile to skip default symlink-forbidding open flag.

---------

Signed-off-by: Jakob Borg <jakob@kastelo.net>

lib/fs/basicfs_unix.go

@@ -17,7 +17,7 @@ import (
 	"syscall"
 )
 
-const alwaysOpenFlags = syscall.O_NOFOLLOW // never open symlinks as the final path component
+const optNoFollow = syscall.O_NOFOLLOW
 
 func (f *BasicFilesystem) CreateSymlink(target, name string) error {
 	name, err := f.rooted(name)