libctf: lowest-level memory allocation and debug-dumping wrappers

libctf: lowest-level memory allocation and debug-dumping wrappers

The memory-allocation wrappers are simple things to allow malloc
interposition: they are only used inconsistently at present, usually
where malloc debugging was required in the past.

These provide a default implementation that is environment-variable
triggered (initialized on the first call to the libctf creation and
file-opening functions, the first functions people will use), and
a ctf_setdebug()/ctf_getdebug() pair that allows the caller to
explicitly turn debugging off and on.  If ctf_setdebug() is called,
the automatic setting from an environment variable is skipped.

libctf/
	* ctf-impl.h: New file.
	* ctf-subr.c: New file.

include/
	* ctf-api.h (ctf_setdebug): New.
	(ctf_getdebug): Likewise.