net/l2tp.sh: Rewrite to address issues of POSIX conformance (and more besides)

Ensure that awk(1) is used portably throughout.

Eliminate the use of ${parameter^^} expansion syntax, which is a bashism.

Delegate netfirc parameter parsing to xargs(1) and awk(1). The potential for
code injection is thus eliminated, to the extent that is currently possible in
netifrc. It also eliminates potential issues pertaining to word splitting and
unintentional pathname expansion.

Add additional sanity checks and increase the rigour of those that exist. For
instance, blank values are no longer permitted and the tunnel_id parameter must
match that of l2tpsession_*, in the case that l2tptunnel_* is defined.

Add additional diagnostic messages while improving the clarity of those that
already existed. This is achieved in some instances by being more precise and,
in others, through the use of English that exhibits greater formality and
consistency. At least one grammatical error was rectified.

Simplify and refine the code in terms of both structure and syntax, and greatly
reduce the number of (local) variables. As a byproduct, all complaints previously
raised by shellcheck have been eliminated, save for the use of local, whose
behaviour is not defined by POSIX. I have not attempted to eliminate the use of
local because, for now, it continues to be used extensively throughout the
netifrc codebase.

Honour the exit status value of ip(8) for the "add" and "del" verbs, rather
than parse STDERR.

Optimise l2tp_post_stop() by refraining from executing ip(8) and awk(8) in the
case that the interface cannot be identifed as a virtual one. Further, do not
attempt to destroy the tunnels associated with an identified session in the
case that the attempt to destroy the session has failed.

Signed-off-by: Kerin Millar <kfm@plushkava.net>
Bug: https://bugs.gentoo.org/890238
Signed-off-by: Sam James <sam@gentoo.org>

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