Search for objective c method definitions

[thj]

Summary

When searching back for function definitions, consider lines starting
with '+' and '-', this allows us to pick up objective methods as well as
c style function definitions.

[bapt]

don't forget to add a unit test

[thj]

• Add tests for finding function names

[pauamma_chezdork.com]

Other than that wording tweak, spelling and grammar in manual page change LGTM.

usr.bin/diff/diff.1

400

[jilles]

usr.bin/diff/tests/functionname_objcm.in

22

Fortunately, this code is never run :) The pointer subtraction is immediate undefined behaviour.