bug#38136: https://github.com/dickmao/gnus

[dick . r . chiang]

RP> That, plus the patch has spurious whitspace changes, and things like the
RP> below, which makes it all harder to read.

I've a mind, of course, to bring non-blocking goodness to the main branch, but
the volume of changes (more than 200 commits and counting at
github.com/dickmao/gnus) requires a social effort that may never reach critical 
mass.

The original patch wishfully believed dynamic-letting globals like
`gnus-summary-buffer` would do an end-run around thread-safety.  But "leakage"
occurs far too often as manifested by "Selected deleted buffer" errors.