common/config.cc: allow integer values to be parsed as SI units
We are allowing this for all and any integer values; that is, OPT_INT,
OPT_LONGLONG, OPT_U32 and OPT_U64.
It's on the user to use appropriate units. For instance, the user should
not use 'E(xabyte)' when setting a signed int, and use his best judgment
when setting options that, for instance, ought to receive seconds.
Fixes: 8265 Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>