net-sim: skip files missing on the card; /api/write accepts a full path

Live-testing against a UICC without the EPS files (no USIM service 85)
and without the Kc files showed the runner aborting when EPSLOCI/Kc were
absent.  write_binary/write_record now take an 'optional' flag: a missing
candidate file is logged as a skip step instead of failing the scenario
(store_epsnsc, real/dummy locations, churn and Kc writes).  Also allow
POST /api/write to select by 'path' like /api/select and /api/read (the
netsim live test needed it to restore the captured file values).

docs/api.md

@@ -350,6 +350,13 @@ Write raw hex data to a file.
 {"name": "EF.ICCID", "fid": "2FE2", "data": "A0A1A2...", "parent_path": ["MF"]}
 ```
 
+A full path may be used instead of name/fid/parent, like `/api/select` and
+`/api/read`:
+
+```json
+{"path": "ADF.USIM/6F7E", "data": "4FE69DE7..."}
+```
+
 For record files:
 ```json
 {"name": "EF.ADN", "fid": "6F3A", "data": "A0A1...", "record_nr": 1, "parent_path": ["MF", "7F10"]}