s2ecmd: Eliminate one Windows-related ifdef
s2ecmd: malloc() + memset(0) ---> calloc()
s2ecmd: Fix parameterless function declarations

This fixes two warnings of the sort
s2ecmd.c:104:6: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
s2ecmd: Add const qualifier to argv to avoid an unnecesary cast
s2ecmd: formatting cosmetics
s2ecmd: fixed function definition

Signed-off-by: Vitaly Chipounov <vitaly.chipounov@epfl.ch>