util: reduce allocation sizes in argconfig_parse()

util: reduce allocation sizes in argconfig_parse()

long_opts stores up to 1 element per option, plus a help element,
and a zerored final element. So it can be allocated with length
options_count + 2 instead of options_count + 3.
short_opts stores up to 3 characters per option, plus 2 help characters,
and a NUL terminator. So it can be allocated with length
options_count * 3 + 3 instead of options_count * 3 + 5.

Also use the two arguments to calloc() instead of multiplying the sizes.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>