Allow passing -C multiple times to load and merge several configuration
files in order. Slice fields are appended; scalar fields (TLS, Log, API,
Metrics, Profiling) are overridden by later files — the same semantics as
the existing mergeConfig() used for CLI+config merging.
- Change Args.CfgFile string to Args.CfgFiles []string
- Extract readConfig() helper from Parse() for single-source loading
- Loop over CfgFiles in Parse(), merging each into the accumulator
- Add Quotas to mergeConfig (was missing from the slice list)
- Add TestParse_MultiFile integration test for two-file merge
Closesgo-gost/gost#150