Bad remote file systems can change mtime unexpectedly,
and then there is the problem that git rebase and similar
operations like to change the files and then change them back,
modifying the mtimes but not the content.
Avoid spurious Put errors on both of those by checking file
content.
(False positive "modified since last read" make the real ones
difficult to notice.)
Move libfmt, libutf into subdirectories of lib9.
Add poll-based socket i/o to libthread, so that we can
avoid using multiple procs when possible, thus removing
dependence on crappy pthreads implementations.
Convert samterm, acme to the single-proc libthread.
Bring libcomplete, acme up-to-date w.r.t. Plan 9 distribution.
