Update determination of iraf and IRAFARCH variables

[olebole]

These variables are now retrieved from the configuration for the different IRAF architectures instead of from /usr/local/bin/cl.

The problems here were that

• /usr/local/bin/cl does not necessarily exist (maybe irafcl, or at different locations)
• the structure of this file changed, there is no longer a unconditional exec statement at the end

Also, there are better ways to find out the iraf variable, which are all tested here:

• Community IRAF puts the IRAF path in ~/.iraf/irafroot resp. /etc/iraf/irafroot,
• personal installations of NOAO and NOIRLab IRAF have an export iraf=/..." statement in ~/.iraf/setup.sh`,
• system installation of NOAO IRAF uses a define in /usr/include/iraf.h for this,
• There a a number of pre-defined ways, like /usr/lib/iraf/, /usr/local/lib/iraf/, /opt/iraf/, /iraf/iraf/ and others

Similarly, to determine IRAFARCH (if not set by environment variable):

• check ~/.iraf/arch (NOAO and NOIRLAB personal installations)
• check the suffix on the standard IRAF bindir (i.e. iraf$bin.linux64)

Closes: #170