the sftp code was one of my first contributions to OpenSSH and it

the sftp code was one of my first contributions to OpenSSH and it
shows - the function names are terrible.

Rename do_blah() to sftp_blah() to make them less so.

Completely mechanical except for sftp_stat() and sftp_lstat() which
change from returning a pointer to a static variable (error-prone) to
taking a pointer to a caller-provided receiver.