process: request_fork: Ensure function parameter is always signed

process: request_fork: Ensure function parameter is always signed

Some architectures, such as armv6, treat char as an unsigned integer
instead of signed. Basically it makes no sense to use char type with
the variable. When it is changed to an int there is no confusion about
the type and it will work better with all architectures.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>