programs: allow processes with a high uid

Since we used signed ints for uids, high ones will appear as
negative values, so we shouldn't exclude this. Otherwise a
user with a high id will get:

ERROR:pkexec.c:718:main: assertion failed: (polkit_unix_process_get_uid (POLKIT_UNIX_PROCESS (subject)) >= 0)
2 jobs for pkexec-highuid in 3 minutes and 41 seconds
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