Minor cleanups

 * Modify the contents of a comment when discussing the ussage of
   'set -e'

 * Do not define true/false (these are historically shell builtins),
   though it may be useful to revisit this as a test condition to see if
   true and false are a builtin in the current shell.

 * Change error message for bad/invalid libraries

 * Prune trailing '/' from SHLIB_PATH
Avoid using 'realpath' as it may not be available.
Define functions to mark imported libraries instead of defining variables.
Move seq to math/seq
Add new 'calc' interface to the math library
Add 'sum()' interface to math library
Start of new regex library
Add simpler match() interface to regex lib