add logn and error checking for invalue eps and epsilon values

Add new logn(x, n [,eps]) builtin to compute logarithms to base n.

Verify that eps arguments (error tolerance arguments that override
the default epsilon value) to builtin functions have proper values.
Previously the eps argument had little to no value checks for
many builtin functions.

Document in help files for builtin functions that take eps arguments,
the LIMIT range for such eps values.

help/sinh

@@ -6,7 +6,7 @@ SYNOPSIS
 
 TYPES
     x		real
-    eps		nonzero real, defaults to epsilon()
+    eps		0 < real < 1, defaults to epsilon()
 
     return	real
 
@@ -22,7 +22,7 @@ EXAMPLE
 
 LIMITS
     unlike sin and cos, x must be real
-    eps > 0
+    0 < eps < 1
 
 LINK LIBRARY
     NUMBER *qsinh(NUMBER *x, NUMBER *eps)
@@ -30,7 +30,7 @@ LINK LIBRARY
 SEE ALSO
     cosh, tanh, sech, csch, coth, epsilon
 
-## Copyright (C) 1999  Landon Curt Noll
+## Copyright (C) 1999,2023  Landon Curt Noll
 ##
 ## Calc is open software; you can redistribute it and/or modify it under
 ## the terms of the version 2.1 of the GNU Lesser General Public License