misc: compiler warning fixes

misc: compiler warning fixes

Fix few compiler warnings. Some of these warnings appeared multiple
times, and the listing bellow is more about which sort of errors
where fixed.

devname.c:87:12: warning: comparison of integers of different signs: 'int' and 'unsigned long'
output.c:389:36: warning: passing 'char **const' to parameter of type 'const char *const restrict *' discards qualifiers in nested pointer types
output.c:611:31: warning: comparison of integers of different signs: 'const unsigned long' and 'int'
stacktrace.c:33:37: warning: unused parameter 'signum'

Signed-off-by: Sami Kerola <kerolasa@iki.fi>