kdump.etc, submit-report.sh: Enhance Steam ID/Account obtaining

Currently we hardcode the account-related VDF file path in kdump,
and expose it in "/etc/default/kdump" - this is unnecessary since
this path is not expected to change nor users to mess with it; thanks
Emil (@xexaxo) for this suggestion.

So, this patch improves things in some ways:

(a) Do not expose VDF path or Valve's server URL in user configurable
file - no reasons for users to mess with that.

(b) Generate "/home" mount point based on DEVNODE, also determine
the username based on "getent'ing" the passwd database. See CAVEAT
below.

(c) Move the VDF parsing to a separate function to clean up the
submit log path on submit-report.sh .

No functional change is expected after this commit.

CAVEAT: Notice that "getent passwd" is *VERY* slow, and if we follow
a generic approach of doing it for UID_MIN..UID_MAX, it takes quite
some time. So, instead we simplify and just query the user 1000; this
might be a bit incomplete, but it's still better than hardcoding a
username as it's done until now.

Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>

README.md

@@ -161,8 +161,7 @@
 #  => The API details: it works by a first POST request to Valve servers,
 #  which, when succeed, returns 3 main components in the response. We use
 #  these values to perform a PUT request with the ZIP compressed file, and
-#  finally a last POST request is necessary to finish the transaction. The
-#  POST requests' URL is present in "/etc/default/kdump".
+#  finally a last POST request is necessary to finish the transaction.
 #  Below, the specific format of such requests:
 #
 #  The first POST takes the following fields: