Standardise spelling of debug info

Debug info being short for debugging information is two separate
words. Therefore, the debuginfo spelling is erroneous. Variables and
defines throughout the code and build system should therefore use
'debug_info' or 'DEBUG_INFO', whereas command-line options and file
names should use the 'debug-info' form. In free form text
(i.e. comments and documentation), 'debug info' should be used, or the
long form 'debugging information'.

Signed-off-by: Antoine Busque <abusque@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>