Introduce XLAT_STYLE_DEFAULT

Introduce XLAT_STYLE_DEFAULT

This will be needed later, with the introduction of user-configurable
xlat style setting (stored in xlat_verbosity variable).

* defs.h (XLAT_STYLE_VERBOSITY_MASK): New macro constant.
(enum xlat_style) <XLAT_STYLE_DEFAULT>: New enumeration entity.
(xlat_verbosity): New external declaration.
(printxvals, printxval_searchn, printxval_search_ex, sprintxval,
sprintflags, printflags64): Use XLAT_STYLE_DEFAULT instead of
XLAT_STYLE_ABBREV.
* strace.c (xlat_verbosity): New variable.
* xlat.c (get_xlat_style): New function.
(printxvals_ex, sprintxval_ex, printxval_searchn_ex, sprintflags_ex,
printflags_ex): Use it.