common: ignore errors from efivarfs mount failures

The existence of /sys/firmware/efi/efivars only indicates that we've
booted the kernel in EFI mode. It does not guarantee that the same
kernel has support for efivarfs. Since this isn't critical, don't fail
the whole api mount.
prepare v12
common: fix errors in valid_number_of_base

Instead of trying to do bit twiddling on what might not even be a
number, just try to cast the subject to the target base.
genfstab: fix deleted file parsing

backslashes in single quotes don't need escaping. We need to match the
literal string \040deleted.
common: avoid need for fallthrough
genfstab: mangle tag values when needed
Add tests for some common functions
README: fix typo
prepare v13