No longer necessary to UN-lock the SMB header before calling smb_addmsghdr(),

this is accomplished with smb_lockmsghdr() and unlockmsghdr() maintaining the
new smb_t.locked variable echoing the messagebase header 'locked' state.
Fixed a couple of error cases in smb_addmsghdr() where it would leave the
smb header in a locked state.
Increased the smblib version to 2.16.