set gcc directive directly to avoid additional dependency in prefix
removed unused mount,umount functions from fs interface
functions types fixed to enable warnings at compilation stage

Before changes it was used the manual type cast at assigning of functions
pointers to structure of pointers aka interface and it's no raised a
warnings if types was mismatched
added zcall 'rename'

This zcall added to be more compliant with filesystems which has own
implementation of 'rename' syscall and that is differ from existed in
glibc; Before change the 'rename' syscall was handled by glibc, now it
handling on user/ZRT side; also it depends on updated glibc.
readlink, symlink, statvfs zcalls added with empty implementations

This commit relies on support of mentioned call by zerovm/glibc
ignore O_TRUNC for emulated channels, add test into glibc check section
avoid intermixing of old and new nvram data, when doing zfork
Merge pull request #141 from YaroslavLitvinov/dev

Dev