Several compiler warnings

[rockdaboot]

There are a bunch of warnings within http2 code, seen with

CFLAGS="-Wall -Wextra -Wformat=2 -Walloc-zero -Walloca -Wbad-function-cast -Wcomments -Wdate-time -Wdouble-promotion -Wduplicated-branches -Wduplicated-cond -Werror-implicit-function-declaration -Wfloat-conversion -Wfloat-equal -Wformat -Wformat-signedness -Wformat-truncation -Winit-self -Winvalid-pch -Wjump-misses-init -Wlogical-op -Wmissing-declarations -Wmissing-format-attribute -Wmissing-include-dirs -Wmissing-prototypes -Wmultichar -Wnested-externs -Wnormalized -Wold-style-definition -Woverlength-strings -Wpedantic -Wpointer-arith -Wredundant-decls -Wscalar-storage-order -Wshift-overflow -Wstrict-prototypes -Wstringop-overflow -Wswitch-default -Wswitch-enum -Wundef -Wunused-const-variable -Wvariadic-macros -Wwrite-strings -Warray-bounds=2 -Wnormalized=nfc -Wshift-overflow=2 -Wunused-const-variable=2 -Wformat-overflow=2 -Wformat-truncation=1 -Wstringop-overflow=2 -Wno-undef -fdiagnostics-color=always -Wno-cast-function-type -Wno-switch-enum" ./configure --enable-http2 --enable-silent-rules
make clean
make

Some are cosmetic, but others are not, like -Wold-style-definition, -Wmissing-prototypes, -Wformat=, -Wunused-variable, -Wdiscarded-qualifiers, -Wsign-compare.