lib/: Don't say 'len' where 'size' is meant

lib/: Don't say 'len' where 'size' is meant

Fixes: 45c6603cc86c ("[svn-upgrade] Integrating new upstream version, shadow (19990709)")
Fixes: 3b7cc053872c ("lib: replace `USER_NAME_MAX_LENGTH` macro")
Fixes: 6be85b0bafb5 ("lib/chkname.c: Use tmp variable to avoid a -Wsign-compare warning")
See-also: 403a2e3771be ("lib/chkname.c: Take NUL byte into account")
See-also: 6a1f45d932c8 ("lib/chkname.c: Support unlimited user name lengths")
Fixes: 95ea61009da8 ("lib/chkname.c: Use precise comment")
Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>
Cc: Tobias Stoeckmann <tobias@stoeckmann.org>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>