Fix minor problems found by static checking.

Fix minor problems found by static checking.

* data.c (pure_write_error):
Use xsignal2, not Fsignal, as Fsignal might return.
* eval.c (set_backtrace_debug_on_exit): Now static.
(backtrace_p, backtrace_top, backtrace_next, record_in_backtrace):
No longer inline.  EXTERN_INLINE is needed only for functions
defined in .h files.  Reindent function header as per GNU style.
(backtrace_p, backtrace_top, backtrace_next):
Mark EXTERNALLY_VISIBLE so they don't get optimized away by the
compiler or linker.  Add extern decls to pacify gcc -Wall.
* frame.c, frame.h (Qgeometry, Qworkarea, Qmm_size, Qframes, Qsource):
Now static.
* frame.c (free_monitors): Define only on platforms that need it.
* nsterm.m (ns_term_init):
* process.c (catch_child_signal):
Don't worry about whether SIGCHLD is defined, as SIGCHLD is
defined on all porting targets these days.
* process.c, process.h (catch_child_signal):
Make it extern only if NS_IMPL_GNUSTEP is defined.