eal: new function to create control threads

eal: new function to create control threads

Many parts of dpdk use their own management threads. Introduce a new
wrapper for thread creation that will be extended in next commit to set
the name and affinity.

To be consistent with other DPDK APIs, the return value is negative in
case of error, which was not the case for pthread_create().

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>