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/sqrt

@@ -15,7 +15,7 @@ TYPES
     For other argument types:
 
     x		real or complex
-    eps		nonzero real
+    eps		0 < real < 1, defaults to epsilon()
     z		integer
 
     return	real or complex
@@ -119,7 +119,7 @@ EXAMPLE
     0 0.0002
 
 LIMITS
-    none
+    0 < eps < 1
 
 LINK LIBRARY
     COMPLEX *c_sqrt(COMPLEX *x, NUMBER *ep, long z)
@@ -130,7 +130,7 @@ LINK LIBRARY
 SEE ALSO
    appr, epsilon
 
-## Copyright (C) 1999,2021  Landon Curt Noll
+## Copyright (C) 1999,2021,2023  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