Fix threaded use of res_ functions.

Commit aab4df0fb51660300559f5f29290709db2f7bfee says that the line with
after res_init() function. Commit
7f74de5d4d6d10baafab4b37bb3d472f5c5f0e8c moves the res_init()
function below the line with #undef _res.

This commit moves res_init() back above #undef _res.

Signed-off-by: Kenneth Soerensen <kenneth.sorensen@spectralink.com>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
Make res_init() thread safe.

res_init() was not atomic, which could give undesired behaviour. Now
res_init() is completely locked under one lock and the locking is
removed from __res_vinit().

Signed-off-by: Kenneth Soerensen <kenneth.sorensen@spectralink.com>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>