Change prefix for man pages from "le_" to "libevent_"
As pointed by @kloczek:
"With only two characters it would be harder to guess under what kind of name are libevent lvl 3 man pages.
Longer name with bash/fix/zsh tab competition should not be a problem :)"
Refs: #1099
Add prefix (le_) for man pages
This will:
- avoid overlapping (since there is util.h.3 page)
- group everything under some prefix (man le_<TAB><TAB> will show everything)
Fixes: #1099
