efivar -a doesn't work, cannot be used to update SecureBoot variables

[SRU Justification]
When using append mode, libefivar's efivarfs_set_variable() opens the target file with flags O_APPEND|O_CREAT, which fails to actually define a read/write mode and therefore the file is opened read-only. This makes it impossible to use efivar to append to variables, which is the only way to update SecureBoot databases.

[Test case]
1. wget -q http://www.uefi.org/sites/default/files/resources/dbxupdate.zip
2. unzip dbxupdate.zip
3. sudo apt install efivar
4. sudo chattr -i /sys/firmware/efi/efivars/dbx-*
5. sudo efivar -n d719b2cb-3d3a-4596-a3bc-dad00e67656f-dbx -a -f /tmp/dbxupdate.bin
6. Confirm that this fails with 'efivar: Invalid argument'.
7. Install efivar and libefivar1 from -proposed
8. Repeat step 5
9. Confirm that this command exits non-zero
10. Confirm that 'mokutil --dbx' shows a significant number of revoked hashes.

[Regression potential]
Since this function has clearly never ever worked, the only regression potential is if someone somewhere is calling this function with a payload that /shouldn't/ be written to nvram, and as a result of fixing this bug they now have junk written in an EFI variable.