fix(ulibc): rename __environ to environ for POSIX compliance

fix(ulibc): rename __environ to environ for POSIX compliance

Replace __environ with environ as the primary environment pointer
symbol throughout ulibc and userland commands. POSIX specifies
'environ' as the standard name (IEEE Std 1003.1).

Files changed:
- user/ulibc/src/stdlib.c: environ as primary symbol, update getenv()
- user/ulibc/src/environ.c: all references updated
- user/ulibc/src/execvp.c: pass environ to execve()
- user/ulibc/include/unistd.h: declare extern environ
- user/cmds/env/env.c: use environ
- user/cmds/printenv/printenv.c: use environ