Adjust nfs4_pwrite and nfs4_pread behavior to be more like C

[scottnuma]

• Implement nfs4_pwrite to return bytes written, else -1 upon error
• struct nfs4 modified to hold the error_num in addition to error_string, as the error number was previously given thru pwrite's return value
• sqllite_nfs4/sqllite_nfs4.c has yet to be compile checked

[scottnuma]

I'm still going through testing process,

nfsv4_read_write_test is still working as previous - this checks pread and pwrite, though I haven't figured out yet how to create an error for testing.

I was able to test the read command for the shell, but not the write. The open write /file.txt command segfaults both before and after the changes, which I think is consistent with the state we left the shell at.

Also still yet to compile or test the ssqlite_nfs4 changes.

[scottnuma]

After some fixes to the shell, I've tested on both read and write and fixed up the documentation of how shell works.

I've now tested with sizes that require nfs4_pread to use multiple rpc_send and transact.