adapt to ${CC_WNO_IMPLICIT_FALLTHROUGH}

Use ${CC_WNO_IMPLICIT_FALLTHROUGH} instead of
the older style more complex expressions.

convert HAVE_GCC == 7 to HAVE_GCC >= 7.

Bump default value of WARNS for modules from 3 to 5, and
explicitly set WARNS for modules that fail with WARNS=5.

Also, turn on -Wno-missing-noreturn for clang for some files.

At the moment, among

Bump default value of WARNS for modules from 3 to 5, and
explicitly set WARNS for modules that fail with WARNS=5.

Also, turn on -Wno-missing-noreturn for clang for some files.

At the moment, among ~ 360 modules,
- 2 (lua and zfs) need WARNS=0
- 1 (solaris) needs WARNS=1
- 136 need WARNS=3 (mostly due to sign-compare)
- 4 need WARNS=4
- others can be compiled with WARNS=5

Discussed on tech-kern.

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be sure to only apply zlib.c's -Wno-error=implicit-fallthrough to GCC 7.
push the setting into the rump and module version too.

Modularize net/zlib so it can be used by the vnd module (and, eventually,
by an opencrypto module).