include: ssp: round out fortification of current set of headers

ssp/ssp.h needed some improvements:
- `len` isn't always a size_t, it may need casted
- In some cases we may want to use a len that

include: ssp: round out fortification of current set of headers

ssp/ssp.h needed some improvements:
- `len` isn't always a size_t, it may need casted
- In some cases we may want to use a len that isn't specified as a
parameter (e.g., L_ctermid), so __ssp_redirect() should be more
flexible.
- In other cases we may want additional checking, so pull all of the
declaration bits out of __ssp_redirect_raw() so that some functions
can implement the body themselves.

strlcat/strlcpy should be the last of the fortified functions that get
their own __*_chk symbols, and these cases are only done to be
consistent with the rest of the str*() set.

Reviewed by: markj
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D45679

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