Package Kit: Fix logic reversal in PackageInfoStringBuilder.

This function is used when serializing a BPackageInfo object, which is not
done very often, and flags are used even less often. But both of these things
are done for the system packages, as they have the "system_package" flag
set, and are processed by "update_package_requires", and so this meant
that the "system_package" flag was turned into the "approve_license" flag.

BPackageInfo::StringBuilder: Fix writing non-string lists

Even a list with only one element needs to be enclosed in {...}.

BPackageInfo::StringBuilder: Fix string escaping

hpkg format: Generalize user/global settings file attributes

* For all identifiers: Rename global settings file to global writable
file. We want to use the respective attribute also for other writable
files, not only settings files.
* User settings file/global writable file info/attribute: Add
isDirectory property/child attribute. This allows declaring global/
user settings directories associated with the package.

hpkg format: Add attributes for declaring users and groups

Move package info string builder to its own header

BPackageInfo::StringBuilder: Fix writing non-string lists

Even a list with only one element needs to be enclosed in {...}.

BPackageInfo::StringBuilder: Fix string escaping

hpkg format: Generalize user/global settings file attributes

* For all identifiers: Rename global settings file to global writable
file. We want to use the respective attribute also for other writable
files, not only settings files.
* User settings file/global writable file info/attribute: Add
isDirectory property/child attribute. This allows declaring global/
user settings directories associated with the package.

hpkg format: Add attributes for declaring users and groups

Move package info string builder to its own header