AbstractMigrate: do not overwrite global signal handlers

AbstractMigrate: do not overwrite global signal handlers

perls 'local' must be either used in front of each $SIG{...}
assignments or they must be put in a list, else it affects only the
first variable and the rest are *not* in local context.

This may cause weird behaviour where daemons seemingly do not get
terminating signals delivered correctly and thus may not shutdown
gracefully anymore.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>