mitigations.7: briefly describe Stack Smashing Protection (SSP)

[emaste]

I'm not sure what makes the most sense for documenting SSP (between this man page and security.7) but there should at least be a reference here.

[kevans]

In D45435#1036428, @emaste wrote:

I'm not sure what makes the most sense for documenting SSP (between this man page and security.7) but there should at least be a reference here.

Oh, hmm, I didn't realize we had this. Looking at the two, I guess it'd make the most sense to just lift the SSP section out of security(7) into mitigations(7), though I think it'd be good to add a more explicit pointer in security(7) to mitigations(7) than just an xref in the SEE ALSO section.
Maybe something like:

For more information on specific vulnerability mitigations implemented in
.Fx ,
see
.Xr mitigations 7 .

[emaste]

Move security.7 SSP description to mitigations.7

[emaste]

Rename section to more generic "stack overflow protection", the section name from security.7

[emaste]

Update correct diff

[emaste]

share/man/man7/security.7

44–51 ↗

(On Diff #139300)

(This intro is extremely dated, and security.7 really needs a holistic rewrite.)

[kevans]

This looks reasonable to me, thanks!