Cache C functions results (all platforms)

[giampaolo]

This is similar to PR #1422 but it applies to all platforms which unnecessarily execute the same routine on each call. Instead the routine should be executed on startup and pushed in the global namespace. Possible candidates (I will update this list as I go):

OSX

• psutil_get_argmax(), used for cmdline() and environ()
• sysctl.proc_cputype, used by memory USS
• getpagesize()

BSD

• getpagesize()

Windows

• GetSystemInfo used to get the pagesize - affected APIs: cpu_count(), Process.memory_maps(), Process.memory_info_ex() (USS field). [DONE]
• psutil_get_nic_addresses(), used by net_io_counters(), net_if_addrs(), net_if_stats()