Do not rely on macros with implicit vars

This commit also provides the benefit that the Label argument
in gen:call/3 is no longer used, as we can access the sender
in the From argument. Therefore, we should be able to change
the Label to something more relevant in the future.
Add note about known issue with -Wrace_conditions

As noted on the erlang-bugs mailing list:
http://erlang.org/pipermail/erlang-bugs/2014-July/004518.html
ssl: Servers may include an empty SNI-extension
Merge branch 'josevalim/jv-genevent' into maint

* josevalim/jv-genevent:
  Do not rely on macros with implicit vars
Merge branch 'ia/ssl/empty-sni-from-server/OTP-12198' into maint

* ia/ssl/empty-sni-from-server/OTP-12198:
  ssl: Servers may include an empty SNI-extension
Merge branch 'legoscia/patch-8/dialyzer-doc-fix' into maint

* legoscia/patch-8/dialyzer-doc-fix:
  Add note about known issue with -Wrace_conditions