add missing va_end in execl* for correcness and static code analyzers

fix new environment always being null with execle

the va_arg call for the argv[]-terminating null pointer was missing,
so this pointer was being wrongly used as the environment pointer.

issue reported by Timo Teräs. proposed patch slightly modified to
simplify the resulting code.

correct variadic prototypes for execl* family

the old versions worked, but conflicted with programs which declared
their own prototypes and generated warnings with some versions of gcc.

initial check-in, version 0.5.0