libcli: New routine symlink_target_path for [MS-SMB2] 2.2.2.2.1.1

libcli: New routine symlink_target_path for [MS-SMB2] 2.2.2.2.1.1

Right now the only user is the user-space symlink following in
smbd. We will use it in libsmb as well to correctly handle
STOPPED_ON_SYMLINK. When trying to upstream that code I found the
previous_slash function incredibly hard to understand.

This new routine makes copy of "const char *_name_in", so that we can
replace previous_slash with a simple strrchr_m. If that's too
slow (which I doubt, this is "only" chasing symlinks) we can always do
something smarter again.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>