eml: speed up common LF-only emails

Emails a *nix MTA are typically LF-only, so we don't need the
complexity of the RE engine when a simple index() works.  We
still need to ensure there's no "\r\n\r\n" before the first
"\n\n", but two calls to index() is still faster than a RE
match.

This gives a 2-5% speedup in some informal tests and saves ~30MB
when scanning a 30MB spam message on newer versions of Perl.
I'll have to diagnose why Perl wastes so much memory doing
RE matches on giant strings, though.

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