Eliminated all build warnings coming from `make check` with
CFLAGS="-Wall -Wextra" (and some "-pedantic") for an x86_64 target (GCC
9 Ubuntu 16.04, GCC 10 Ubuntu 20.04). Except for the "Implement
get_list_addr(), please." beg.
This was mostly tagging various parameters as "maybe unused", but there
were a few other changes like assigning to the correct member of
sa_handler and casting things to the right type.
This will hopefully reduce the noise in builds.
The calls to mincore() or msync() are not checking for actual accessibility
this could lead to SIGSEGV if the address from a mapped page with the
PROT_NONE property occurs on the stack.
Hence an attempt to write one byte from the checked address to a pipe will
fail if the address is not readable.
