Various fixes and improvements

Commits that need some attention:
1. xseg_posix: Simplify posix_allocate function
2. xseg_pthread: Simplify posix_allocate function
3. xseg_user: Remove xseg_snprintf wrapper
4. Remove superfluous __KERNEL__ definitions
5. xseg: Remove superfluous NULL definition
6. xseg_posix: Remove fixed address memory mapping
7. cmake: Remove global symbol exports