Relax detection of images for strip detection

It is perfectectly fine to have an image cited in an HTML body similar to:

<img width=909 height=302 src=cid:image002.jpg@01CF5426.BF5A72A0 alt=image>

Which may or may not have quoted @src attribute, and may very well have an
at sign (@) somewhere in the attribute text. The previous regular expression
would not match such a @src attribute.
validation: Fixup a few email address validations
Fixup merging of email recipients

This stems from a confusing similarity between the + operator for arrays
and array_merge() in php. Adding arrays will ignore items in the RHS where
keys are present in the LHS. Therefore, when adding numerically indexed
arrays together, only items on the RHS that have a key higher than the
greatest key on the LHS will be included.