Retrieve password for user from specified file. The buffer key
contain the password if this function is successful. The caller is
responsible for deallocating it.
The file should be on the UoW "MD5 Based Authentication" format,
which means it is in text format with comments denoted by # first
on the line, with user entries looking as "usernameTABpassword".
This function removes CR and LF at the end of lines before
processing. TAB, CR, and LF denote ASCII values 9, 13, and 10,
Return GSASL_OK if output buffer contains the
password, GSASL_AUTHENTICATION_ERROR if the user could not be
found, or other error code.