getgroups.2: Clarify, mention ascending order, add SECURITY CONSIDERATIONS

[olce]

Summary

Clarify and be more precise about the behavior of getgroups(2), in
particular with respect to 'gidsetlen'.

Prefer a terminology referring to POSIX terms, i.e., use "supplementary
groups" instead of "group access list".

Say that getgroups(2) reports the supplementary groups in strictly
ascending order and returns the cardinal of the set they form (and
mention this has been the case since FreeBSD 14.3).

Add a new SECURITY CONSIDERATIONS section contrasting the new behavior
after commit 9da2fe96ff2e ("kern: fix setgroups(2) and getgroups(2) to
match other platforms") with the historical one.

While here, fix some style.

Sponsored by: The FreeBSD Foundation

[gbe]

LGTM from manpages

[olce]

• Emphasize that the supplementary groups form a set, and that the number of elements we return is its cardinal.
• SECURITY CONSIDERATIONS: Emphasize that the effective GID is not necessarily part of the set, and that initgroups(3) and setgroups(2) differ in this respect.
• "current process" => "calling process"