lint: reword lint2 warnings, reduce punctuation

The previous diagnostics used unusual punctuation, for example by placing
spaces inside parentheses, as well as using space-space-tab and
double-colon as separators. Convert these diagnostics into readily
readable prose, and mark them clearly as coming from lint2, so that they
can be related to the tests in /usr/tests/usr.bin/xlint/lint2/msg_???.ln.

This change means that usr.bin/error no longer recognizes these messages
as coming from lint2, but that can be fixed later if necessary.

lint: fix type name of prototype function in lint2 messages

Since tyname.c 1.42 from 2021-06-28, when the type names for '(void)'
prototypes was fixed for lint1 but not for lint2.

tests/lint: demonstrate wrong type name for prototype function

branches: 1.2.4;
tests/lint: add more tests for lint2

tests/lint: add test skeletons for messages from lint2