scripts/checkpatch.pl: Enforce multiline comment syntax

scripts/checkpatch.pl: Enforce multiline comment syntax

We now require Linux-kernel-style multiline comments:
    /*
     * line one
     * line two
     */

Enforce this in checkpatch.pl, by backporting the relevant
parts of the Linux kernel's checkpatch.pl. (The only changes
needed are that Linux's checkpatch.pl WARN() function takes
an extra argument that ours does not, and the kernel has a
special case for networking code we don't want.)"

The kernel's checkpatch does not enforce "leading /* on
a line of its own, so that part is unique to QEMU's checkpatch.

Sample warning output:

WARNING: Block comments use a leading /* on a separate line
#34: FILE: hw/intc/arm_gicv3_common.c:39:
+    /* Older versions of QEMU had a bug in the handling of state save/restore

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>