initgroups.3: Clarify that ENOMEM is a possible value for 'errno'

The current phrasing could be interpreted as meaning that initgroups(3)
could return ENOMEM instead of -1, which it actually did until I fixed
it in commit 0b018cfd81d8 ("initgroups(3): Fix return value on
allocation failure").

While here, mention setgroups(2) as a system call rather than a library
function.

MFC with: 0b018cfd81d8 ("initgroups(3): Fix return value on allocation failure")
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