make-dissectors-reg: optimize by factoring out the loops

Instead of calling the grep/sed pipelines for each file, build the
list of files in the beginning and call each pipeline only once,
passing the list to the first grep.

This results in a massive speedup in Cygwin; in my test, the time
it takes to run make-dissector-reg . dissectors packet-*.c in dissectors/epan
is reduced from ~116 to ~3 seconds. I also tried it on NetBSD, where
the time do to the same goes from ~6 to ~0.5 seconds.

Amend makefile comments to elide mentions of invoking multiple processes
per file.

Change-Id: Iad441e7d2b6cc3669dada57646e2f8f6b987fd34
Reviewed-on: https://code.wireshark.org/review/2826
Reviewed-by: Anders Broman <a.broman58@gmail.com>

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