Support append

[Chenlibo]

1. fixed bugs in nfs4_append and nfs4_change_properties
2. modified nfs4_append so that it returns the number of bytes appended and updated corresponding code in shell.c and nfsv4_read_write_test.c
3. combined nfs4_pwrite and nfsv4_append into nfs4_write in C library
4. added tests for append mode in sfs_test.py
5. added encoding argument in sfs.open to potentially support encode and decode when buffering is disabled