abuild: add subpkgver capabilities

Adds the option to set custom subpackage version.

As with arch, defined within the subpackages array using the following syntax: subpkgname:subpkgsplit:subpkgarch:subpkgver.

Also brings the following improvements to subpkg_set function:

• subpkgarch can be set without setting subpkgsplit
• proper checking if subpkgarch is actually supported arch using new is_arch function
• proper checking if subpkgver is actually version information by checking if variable contains '.'
• as long as syntax order is respected, any extra variables can be undefined

Possible improvements:

• is_arch relies on hardcoded local supportedArch array. It'd be nice if this was sourced from somewhere else so that future (un)supported architectures can automatically be detected.
• subpkg_set certainly has room for optimizations. For readability, things were kept as-is to make sure there are no syntax bugs.
• while default_sanitycheck was updated to check subpkgver, likely other aport sanity check scripts need updating
• ideally subpkg_set should not need an explicit order to the arguments. With 2 out of 3 arguments being detectable, it would be possible for subpkg_set to go through all of the arguments and be able to read which arg is associated to which option. This would complexify the function, thus move away from KISS principles, which is why I didn't go this direction.

Accompanies lua-aports!4 Fixes part of: #10060