Update cwrapper and tests per review comments
    
    * libltdl/config/ltmain.m4sh (func_emit_cwrapperexe_src):
    Update comments. Initialize program_name. Eliminate _LENGTH
    variables for string constants. In debug mode, print a
    banner with known content before any other output.
    (func_emit_cwrapperexe_src:main): Use strcmp rather than
    strncmp, where safe.
    (func_emit_cwrapperexe_src:lt_debugprintf): Modify signature
    and implementation to conform to GCS.
    (func_emit_cwrapperexe_src:lt_fatal): Ditto.
    (func_emit_cwrapperexe_src:lt_error_core): Ditto.
    (func_emit_cwrapperexe_src): Update all callers to lt_fatal
    and lt_debugprintf.
    * tests/cwrapper.at: Clarify comments. Avoid unnecessarily
    copying libtool. Adjust debug tests to search for wrapper
    script banner message.