Hide process_lock() into convenient wrappers, remove unnecessary process_lock() usage.

There are two changesets in the pull request:
1) first, process_lock()/process_unlock() are removed around system calls
2) second, several wrappers for stdio functions are added that do the locking only on libcs other than glibc, eglibc and uClibc.

I've asked about a more convenient way to handle locking in [1] and got reply [2], that wrappers for stdio functions make sense.
But, please, note, that these changes are only compile-tested.

[1] https://github.com/lxc/lxc/pull/101#issuecomment-29609420
[2] https://github.com/lxc/lxc/pull/101#issuecomment-29622513