openvpn: fix quoting and deprecated option filtering

openvpn: fix quoting and deprecated option filtering

This patch fixes two issues in the netifd protocol script:

1. Fix logic error in deprecated option filtering:
   Previously, ${f%%:*} was called before checking for the deprecated
   flag (:d). This stripped the suffix and made the check [ "${f#*:}" = "d" ]
   always fail. The cleaning of $f is now deferred until after this check.

2. Improve parameter quoting for specific options:
   - Adds single quotes to --push and --push-remove parameters to handle
     spaces (e.g., "route 10.0.0.0 255.255.255.0").
   - Unifies quoting for 'file' type options to improve shell safety.
   - Refactors the build logic using a case statement for better
     extensibility.

Signed-off-by: Chen Minqiang <redacted>