src/elflint.c: fix printing of unknown flags

src/elflint.c: fix printing of unknown flags

before the change section_flags_string() ignored unknown section
flags: snprintf() did write numeric value into buffer, but
"*cp = '\0'" negated the effect.

The change advances the 'cp' pointer'.

While at it add a '|' separator between known and unknown flags.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>