Implement svn_cstring_skip_prefix().

* subversion/libsvn_subr/string.c
* subversion/include/svn_string.h
  (svn_cstring_skip_prefix): New.

* subversion/tests/libsvn_subr/string-test.c
  (test_string_skip_prefix): New test for svn_cstring_skip_prefix().
  (test_funcs): Add test_string_skip_prefix test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1612823 13f79535-47bb-0310-9956-ffa450edef68