improvings some of low-level functions by using their libc counterpart

Many uwsgi utils function are sub-optimals and replacing them by their standard lib counterpart results in less code and may results in performance improvement.
Some commits also leverage advantages given by the operating system, and prevent misuse of these advantages, like re-zeroing already zeroed memory area (which also discard the benefit of virtual memory handling of most OS, like viurtal zeroing and lazily allocation ; using memset forces them to brutally actually allocate it, in one time)(see: http://landley.net/writing/memory-faq.txt)