getgrouplist.3: Insist on the value returned in 'ngroups'; Minor changes

Emphasize the peculiar treatment of 'ngroups', through which the
effective group list's size is returned, even if the passed allocated
size is not big enough, in which case the output 'groups' array is
completely filled with the effective group list's first elements.

Use the precise POSIX term for the group associated to the user in the
password database ("initial numerical group ID"), consistently with what
was done in initgroups.3.

Use "length" instead of "size" to refer to the number of elements of an
array (in contrast with its actual size in bytes).

Perform some minor simplifications on top of the last rework.

MFC after: 1 hour
Sponsored by: The FreeBSD Foundation

(cherry picked from commit f5544556754ec6b33c1d13c7648a354fef645b2f)