sysroot: Support boot counting for boot entries

sysroot: Support boot counting for boot entries

Add support for boot counting for bootloader entries [1].
The boot counting data is stored in the name of the boot loader entry.
A boot loader entry file name may contain a plus (+) followed by a number.
This may optionally be followed by a minus (-) followed by a second number.
The dot (.) and file name suffix (conf or efi) must immediately follow.

The feature is enabled via sysroot configuration:
[sysroot]
boot-counting-tries=3

Testing:
$ ostree admin deploy 91fc19319be9e79d07159303dff125f40f10e5c25614630dcbed23d95e36f907
Copying /etc changes: 2 modified, 3 removed, 4 added
bootfs is sufficient for calculated new size: 0 bytes
Transaction complete; bootconfig swap: yes; bootversion: boot.0.1, deployment count change: 1

$ ls /boot/loader/entries
ostree-1.conf  ostree-2+3.conf

[1] https://uapi-group.org/specifications/specs/boot_loader_specification/#boot-counting
Signed-off-by: Igor Opaniuk <igor.opaniuk@foundries.io>
Signed-off-by: Colin Walters <walters@verbum.org>