Introduce memory allocation wrappers

Introduce memory allocation wrappers

Introduce wrappers to the following functions that do memory allocation:
malloc, calloc, realloc, strdup.

This commit is a follow-up to the related discussions in strace-devel ML:
http://sourceforge.net/p/strace/mailman/message/33618180/
http://sourceforge.net/p/strace/mailman/message/33733470/

* defs.h (xmalloc, xcalloc, xreallocarray, xstrdup): New prototypes.
* xmalloc.c: New file.
* Makefile.am (strace_SOURCES): Add it.
* count.c (count_syscall, call_summary_pers): Use xcalloc.
* desc.c (decode_select): Use xmalloc.
* dirent.c (sys_getdents, sys_getdents64): Likewise.
* net.c (sys_recvmmsg): Use xstrdup.
* pathtrace.c (storepath): Use xreallocarray.
(pathtrace_match): Use xmalloc.
* strace.c (die_out_of_memory): Move to xmalloc.c.
(expand_tcbtab): Use xcalloc and xreallocarray.
(startup_child): Use xstrdup.
(init): Use xmalloc, xcalloc, and xstrdup.
* syscall.c (reallocate_qual): Use xreallocarray.
(qualify): Use xstrdup.
* unwind.c (unwind_tcb_init): Use xmalloc.
(build_mmap_cache): Use xcalloc, xreallocarray, and xstrdup.
(get_symbol_name): Use xreallocarray.
(stacktrace_walk, queue_put): Use xmalloc.
* util.c (printstr): Use xmalloc.
* vsprintf.c (strace_vfprintf): Likewise.