Use int type to store result from isc_commandline_parse()

Use int type to store result from isc_commandline_parse()

The C standard actually doesn't define char as signed or unsigned, and
it could be either according to underlying architecture.  It turns out
that while it's usually signed type, it isn't on arm64 where it's
unsigned.

isc_commandline_parse() return int, just use that instead of the char.

(cherry picked from commit 8153729d3a59eafa7e02068f239d9a999d302b99)