Fix a rare buffer underun bug when present("1") is called.

Also added a comprehensive test.
Add a crasher test for present("1"), only shows under valgrind.
Update efun doc for present.
Deleted text and html version of doc. Migrate nroff version to /doc.
Adding back -flto flag.
Remove a debug message before dump backtrace.
Release 3.0-alpha6.1
Merge pull request #122 from sunyc/master

Additional CL for fluffos-3.0-alpha6.1