canonicalize: add support for not resolving symlinks

canonicalize: add support for not resolving symlinks

This will initially be used by a new coreutils realpath command.

* lib/canonicalize.h: Add the CAN_NOLINKS flag to
indicate we don't want to follow symlinks.  Also
provide CAN_MODE_MASK to aid setting these existing
mutually exclusive values.
* lib/canonicalize.c (canonicalize_filename_mode):
Extract the flags from can_mode parameter, which
are currently just used to select between stat()
and lstat().  Also ensure that mutually exclusive
values are flagged immediately as invalid.
* tests/test-canonicalize.c: Verify symlinks are
not followed, and that invalid flag combinations
are diagnosed.