lvcreate/lvresize: the --size option accepts signed values

lvcreate/lvresize: the --size option accepts signed values

There was confusion in the code about whether or not the
--size option accepted a sign.  Make it consistent and clear
that it does.

This exposes a new problem in that an option can only
accept one value type, e.g. --size can only accept a
signed number, it cannot accept a positive or negative
number for some commands and reject negative numbers for
others.

In practice, lvcreate accepts only positive --size
values and lvresize accepts positive or negative --size
values.  There is currently no way to encode this
difference.  Until that is fixed, the man page output
is hacked to avoid printing the [+|-] prefix for sizes
in lvcreate.