Fix non _REENTRANT build.

Improve and simplify implementation of *env(3) functions:
- Use RB tree to keep track of memory allocated via setenv(3) as
suggested by Enami Tsugutomo in private e-mail.
This simplifies the code a lot as we no longer need to keep the size
of "environ" in sync with an array of allocated environment variables.
It also makes it possible to free environment variables in unsetenv(3)
if something has changed the order of the "environ" array.
- Fix a bug in getenv(3) and getenv_r(3) which would return bogus
results e.g. for " getenv("A=B") " if an environment variable "A"
with value "B=C" exists.
- Clean up the internal functions:
- Don't expose the read/write lock for the environment to other parts
of "libc". Provide locking functions instead.
- Use "bool" to report success or failure.
- Use "ssize_t" or "size_t" instead of "int" for indexes.
- Provide internal functions with simpler interfaces e.g. don't
combine return values and reference arguments.
- Don't copy "environ" into an allocated block unless we really need
to grow it.

Code reviewed by Joerg Sonnenberger and Christos Zoulas, tested by
Joerg Sonnenberger and me. These changes also fix problems in
zsh 4.3.* and pam_ssh according to Joerg.

Use a bit array to keep track of malloced environment entries so we can
free them.

compat core reorg.