overhaul environment functions

Rewrite environment access functions to slim down code, fix bugs and
avoid invoking undefined behavior.

* avoid using int-typed iterators where size_t would be correct;
* use strncmp instead of memcmp consistently;
* tighten prologues by invoking __strchrnul;
* handle NULL environ.

putenv:
* handle "=value" input via unsetenv too (will return -1/EINVAL);
* rewrite and simplify __putenv; fix the leak caused by failure to
  deallocate entry added by preceding setenv when called from putenv.

setenv:
* move management of libc-allocated entries to this translation unit,
  and use no-op weak symbols in putenv/unsetenv;

unsetenv:
* rewrite; this fixes UB caused by testing a free'd pointer against
  NULL on entry to subsequent loops.

Not changed:
Failure to extend allocation tracking array (previously __env_map, now
env_alloced) is ignored rather than causing to report -1/ENOMEM to the
caller; the worst-case consequence is leaking this allocation when it
is removed or replaced in a subsequent environment access.

Initially UB in unsetenv was reported by Alexander Cherepanov.
Using a weak alias to avoid pulling in malloc via unsetenv was
suggested by Rich Felker.

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