efivar: Attempt to fix setting/printing/deleting EFI vars with '-' in their name

Due to how we're parsing UUIDs, we were disallowing setting, printing or
deleting any UEFI variable with a '-' in it when you attempted to do that
operation with the exact name (wildcard reporting was unaffected). Fix the
parser to loop over all the dashes in the name and only give up when all
possible matches are exhausted.

Reviewed by: markj@
Sponsored by: Netflix, Inc
Differential Revision: https://reviews.freebsd.org/D29620