Fix buildkernel for lex/yacc upgrades, too.
hotplugd(8): Remove some compat defines and add comments about it.
m4/ngctl/ppp: Silence gcc47 -Wcast-qual warnings.
libdmsg: last{dist,rnss} are used with REQUIRE_SYMMETRICAL only.

While here, also remove two unused variables.
bmake: Mark a return value unused.
libfetch: Remove an unused variable.
libdmsg: Mark some return values unused.