MINOR: http: export some of the HTTP parser macros

MINOR: http: export some of the HTTP parser macros

The two macros EXPECT_LF_HERE and EAT_AND_JUMP_OR_RETURN were exported
for use outside the HTTP parser. They now take extra arguments to avoid
implicit pointers and jump labels. These will be used to reimplement a
minimalist HTTP/1 parser in the H1->H2 gateway.