all: fix IPsec capitalization

"IPSec" was still present in very few places. Let us be coherent and
always use the preferred "IPsec" capitalization.

Quote from RFC 4301:
'The spelling "IPsec" is preferred and used throughout this and all
related IPsec standards.  All other capitalizations of IPsec (e.g.,
IPSEC, IPSec, ipsec) are deprecated.'
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