pvscan: allow use of noudevsync option

pvscan: allow use of noudevsync option

When pvscan is used to activate a VG via an
asynchronous service (i.e. lvm2-pvscan), there
is no requirement that the command wait for
udev to create device nodes before returning.

It's possible that waiting for udev is slow
enough to cause the service running the command
to time out.  So, allow the --noudevsync option
to be given to pvscan to skip waiting for udev.

(This commit is not changing the lvm2-pvscan
service itself to use --noudevsync.)

Still unknown is whether there are any complex
LV activation cases in which lvm itself requires
access to a device node, in which case the udev
wait could be needed by lvm itself.

(When running an activation command directly
from the command line, it's generally expected
that the activated LVs are ready to use when
the command is finished, so lvm waits for
udev to finish creating the dev nodes.)