sed(1): Sync with FreeBSD (adds -u switch)

This is a minor sync with FreeBSD.  The -r option, an alias of -E, was
recently added.  This adds "-u" which makes the output unbuffered.  There
are minor tweaks to newline and file handling.
csu: isolate gcc-specific flags

Limiting gcc-specific flags to CCVER that begins with "gcc"  allows world
to build on clang without NO_WARNS defined.
sed(1): Fix implicit definition of getline error