kvprintf(): Fix '+' conversion handling

For example, printf("%+i", 1) prints "+1". However, kvprintf() did
print just "1" for this example. According to PRINTF(3):

A sign must always be placed before a number produced by a signed
conversion.

For "%+r" radix conversions, keep the "+" handling as it is, since this
is a non-standard conversion. For "%+p" pointer conversions, continue
to ignore the sign modifier to be in line with libc.

This change allows to support the ' conversion modifier in the future.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1310