resolved: add configurable DNS cache size

resolved: add configurable DNS cache size

Add CacheSize= option to [Resolve] section of resolved.conf to allow
configuring the maximum number of entries in the per-scope DNS cache.
The default remains 4096 entries. Setting this to 0 disables caching
(similar to Cache=no).

CacheSize= is only read when Cache=yes or Cache=no-negative. When
Cache=no, caching is fully disabled regardless of CacheSize=.

Changes:
- Add cache_size field to Manager struct
- Parse CacheSize= from resolved.conf via gperf
- Thread cache_size through dns_cache_put() and helper functions
- Replace hard-coded CACHE_MAX with the configurable cache_size
- When cache_size is 0 or Cache=no, flush cache and skip caching
- Add man page documentation for the new option
- Add unit tests for cache size enforcement

Co-developed-by: Claude <claude@anthropic.com>