MINOR: sample: store location for fetch/conv via initcalls

MINOR: sample: store location for fetch/conv via initcalls

Now keywords are registered with an exec_ctx and this one is passed
when calling ->process. The ctx is of type INITCALL when passed via
an initcall where we know the file name and line number.

This was tested with and extra "malloc(15)" added in smp_fetch_path()
which shows that it works:

  $ socat /tmp/sock1 - <<< "show profiling memory"|grep via
           Calls         |         Tot Bytes           |       Caller and method  [via]
      1893399           0       60592592              0|         0x78b2ec task_run_applet+0x3339c malloc(32) [via initcall @http_fetch.c:2416]