cleanup: use internal memory allocation wrapper functions everywhere

cleanup: use internal memory allocation wrapper functions everywhere

The "x"-prefixed versions of strdup, malloc, etc. will check whether the
allocation was successful and terminate the process otherwise.

A few uses of malloc were left alone since they already implemented a
graceful path of failure or were in a quasi external library like xdiff.

Additionally, the call to malloc in compat/win32/syslog.c was not modified
since the syslog() implemented there is a die handler and a call to the
x-wrappers within a die handler could result in recursion should memory
allocation fail.  This will have to be addressed separately.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>