udev: accept IMPORT{}= without any fuss

Udev logs are full of messages about wrong operator type:
...
Reading rules file: /usr/lib/udev/rules.d/60-persistent-storage.rules
/usr/lib/udev/rules.d/60-persistent-storage.rules:30 IMPORT key takes '==' or '!=' operator, assuming '=='.
/usr/lib/udev/rules.d/60-persistent-storage.rules:30 IMPORT key takes '==' or '!=' operator, assuming '=='.
/usr/lib/udev/rules.d/60-persistent-storage.rules:30 IMPORT key takes '==' or '!=' operator, assuming '=='.
/usr/lib/udev/rules.d/60-persistent-storage.rules:30 IMPORT key takes '==' or '!=' operator, assuming '=='.
/usr/lib/udev/rules.d/60-persistent-storage.rules:30 IMPORT key takes '==' or '!=' operator, assuming '=='.
/usr/lib/udev/rules.d/60-persistent-storage.rules:30 IMPORT key takes '==' or '!=' operator, assuming '=='.
/usr/lib/udev/rules.d/60-persistent-storage.rules:54 IMPORT key takes '==' or '!=' operator, assuming '=='.
/usr/lib/udev/rules.d/60-persistent-storage.rules:57 IMPORT key takes '==' or '!=' operator, assuming '=='.
/usr/lib/udev/rules.d/60-persistent-storage.rules:60 IMPORT key takes '==' or '!=' operator, assuming '=='.
/usr/lib/udev/rules.d/60-persistent-storage.rules:63 IMPORT key takes '==' or '!=' operator, assuming '=='.
/usr/lib/udev/rules.d/60-persistent-storage.rules:66 IMPORT key takes '==' or '!=' operator, assuming '=='.
/usr/lib/udev/rules.d/60-persistent-storage.rules:67 IMPORT key takes '==' or '!=' operator, assuming '=='.
/usr/lib/udev/rules.d/60-persistent-storage.rules:93 IMPORT key takes '==' or '!=' operator, assuming '=='.
/usr/lib/udev/rules.d/60-persistent-storage.rules:107 IMPORT key takes '==' or '!=' operator, assuming '=='.
/usr/lib/udev/rules.d/60-persistent-storage.rules:110 IMPORT key takes '==' or '!=' operator, assuming '=='.
/usr/lib/udev/rules.d/60-persistent-storage.rules:113 IMPORT key takes '==' or '!=' operator, assuming '=='.
Reading rules file: /usr/lib/udev/rules.d/60-persistent-v4l.rules
/usr/lib/udev/rules.d/60-persistent-v4l.rules:7 IMPORT key takes '==' or '!=' operator, assuming '=='.
/usr/lib/udev/rules.d/60-persistent-v4l.rules:9 IMPORT key takes '==' or '!=' operator, assuming '=='.
/usr/lib/udev/rules.d/60-persistent-v4l.rules:16 IMPORT key takes '==' or '!=' operator, assuming '=='.
...

The warning was downgraded in f0beb6f8, but I
think it should be removed altogether. IMPORT{program}="asdf" seems like an
obvious way to write this, and people don't expect to have to write "==".
So let's just allow any operator.

(cherry picked from commit b6a80b83)