Add const to char* parameters that are not mutated

[TedLyngmo]

Marked read-only string parameters as const in the public API:

• expand_cb typedef: name parameter
• clispec_parse_str: treename parameter
• clispec_parse_file: name and treename parameters
• cligen_parseerror: error string parameter (s)

Updated implementations and example callbacks to match.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.30%. Comparing base (bd58d3d) to head (161c437).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #137   +/-   ##
=======================================
  Coverage   53.29%   53.30%           
=======================================
  Files          23       23           
  Lines        8038     8039    +1     
  Branches     1870     1870           
=======================================
+ Hits         4284     4285    +1     
  Misses       2701     2701           
  Partials     1053     1053           

Files with missing lines	Coverage Δ
cligen_file.c	72.72% <ø> (ø)
cligen_syntax.c	34.31% <ø> (ø)
cligen_tutorial.c	63.48% <ø> (ø)

... and 1 file with indirect coverage changes

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